Overall, this is a mixture of true, false, and unverifiable claims.
First things first: I'm going into this highly skeptical, because part of the claim being made is extreme to me. The OP writes,
Prominent detractors, including scientists and journalists went missing or were found dead, in horrific circumstances. One’s body was found in a terrible condition, inside a dumpster, if I recall.
If not, these scientist and journalists suddenly became BPs biggest supporters, looking like stepford wives on Fox News.
I wanted to take pictures of the spill in Louisiana before the disperment. I was a minor at the time, and even still my father made it clear, “they’ll still arrest you and I’ll never see you again.” Arrest me? How? WHY? What’s illegal about photographing a beach?
NEVER FORGET it was Obama that declared anyone - journalist or any other civilian who attempted to document the spill, the plight of the cleaners or the environmental degradation - would be detained by the Department of Homeland Security.
However, there is surprisingly some truth in here.
No verifiable murders; half-hearted corporate coverup
The Guardian reports that, far from a conspiracy of silence, there was open dissent inside the EPA, and even gave us some names.
The EPA issued a report on Monday, based on a study of how much of the mixture was needed to kill a species of shrimp and small fish, just two of the 15,000 types of marine life in the Gulf. The EPA test did not address medium- or long-term effects, or reports last week that dispersants were discovered in the larvae of blue crab, entering the food chain.
Hugh Kaufman, a senior EPA policy analyst, dismissed the tests as little more than a PR stunt. "They are trying to spin this limited piece of information to make it look like dispersants are safe and that the Corexit dispersant is safe."
The controversy surrounding EPA's role in the oil spill marks a turning point for the Obama administration, which came to power vowing to repair the frayed relationship between scientists and government under George Bush and promising a new era of transparency.
Nine leading scientists have written a public letter calling on BP and the Obama administration to release all scientific data related to the spill, including wildlife death. "Just as the unprecedented use of dispersants has served to sweep millions of gallons of oil under the rug, we're concerned the public may not get to see critical scientific data until BP has long since declared its responsibility over," said Bruce Stein of the National Wildlife Federation.
The National Wildlife Federation is one of the largest environmentalist groups in the US. Hugh Kaufman, Bruce Stein, and another prominent critic named Ron Kendall are all still with us on this mortal plane, and as far as I can tell were consistent in their criticism of Corexit after the disaster.
It is true that the makers of Corexit did their best to prevent scientific testing of the substance. Furthermore, it is true that the dispersant has proven dangerous -- at least according to a widely cited study: "Toxicity of dispersant Corexit 9500A and crude oil to marine microzooplankton." Ecotoxicology and environmental safety 106 (2014): 76-85.
Our results indicate that Corexit 9500A is highly toxic to microzooplankton, particularly to small ciliates, and that
the combination of dispersant with crude oil significantly increases the toxicity of crude oil to
microzooplankton. The negative impact of crude oil and dispersant on microzooplankton may disrupt
the transfer of energy from lower to higher trophic levels and change the structure and dynamics of
marine planktonic communities.
Nobody involved in this 2014 study has been murdered, as far as I can tell. It has suffered the comparably sad fate of being generally overlooked by the news media.
In the comments to the OP, it's claimed that you can find evidence for the OP's claims in old episodes of Democracy Now. Antonia Juhasz, author of the 2011 book Black Tide: The Devastating Impact of the Gulf Oil Spill, did indeed go on Democracy Now in 2015 and talked about the long lasting harm done by the dispersant, yet she is still with us to this day. Her book does not seem to record any mysterious deaths associated with the spill, nor does it mention critics of Corexit who became defenders. It does mention, citing an NOAA study, that the dispersant polluted the air and water just as much as crude oil itself.
OP refers to a "body in a dumpster". This is John P. Wheeler III, whose death is certainly mysterious, but it is not clear what his connection is to the spill. He was last seen "wandering" near the DuPont offices... which were also the offices of his employer, Mitre Corporation.
This reference seems to come from a list of deaths of people supposedly linked to the spill, but lists of "suspicious" deaths are easy to make up for an event of this magnitude. To me, the weirdest entry on this list is not Wheeler but Ted Stevens, the "internet is a series of tubes" guy. Stevens doesn't seem like a whistleblower type to me. He died in a plane crash in rural Alaska, but not everyone on that plane died.
I am in agreement with Kamil's answer that no reporter deaths have been linked to the spill, and there is no good evidence that whistleblowers were killed either.
Photography prevented; sick employees silenced; DHS involvement
The less extreme claims, that photography was prevented and sick employees were unable to speak to media, are true, according to the New York Times, which for instance found the following:
In a separate incident last week, a reporter and photographer from The Daily News of New York were told by a BP contractor they could not access a public beach on Grand Isle, La., one of the areas most heavily affected by the oil spill. The contractor summoned a local sheriff, who then told the reporter, Matthew Lysiak, that news media had to fill out paperwork and then be escorted by a BP official to get access to the beach.
BP did not respond to requests for comment about the incident.
"For the police to tell me I needed to sign paperwork with BP to go to a public beach?" Mr. Lysiak said. "It's just irrational."
In another incident, mentioned in the 2011 book A Sea in Flames: The Deepwater Horizon Oil Blowout, a BP employee answered the phones at an FAA office to deny reporters access to airspace over the cleanup areas. The FAA claimed that the BP employee had wandered into their office.
One reporter who was on the scene goes into greater detail.
Shortly before I arrived in Louisiana, in early July, Admiral Allen responded to the uproar of protests over the media blockade by going on national television and officially assuring the media that they would have “uninhibited access” to all clean-up areas. In actuality, as I and other members of the media soon discovered, the blockade in the Gulf only tightened. Flyover permits were revoked, and aerial access was tightly restricted. Even accredited media flights were denied permits to fly below 900 feet over clean-up areas. All photography or filming on public beaches was prevented. National Guardsman blocked Anderson Cooper’s CNN team from filming oil-damaged birds at Fort Jackson, Louisiana. Even the New York Times and filmmakers from PBS were refused permission to fly over “Ground Zero.” The blockade was so tight that workers for BP’s so-called “Vessels of Opportunity” program were contractually threatened with immediate job loss if they so much as spoke with anyone in the media.
I found that the main bird-cleaning facilities at Fort Jackson were closed to the media except for a couple of days a week. Sick workers were contractually forbidden to speak to journalists.
I cannot find any much else about the sick workers, besides anecdotal evidence like that reported in the books above, or this interview offered to RT.
A film crew was prevented from filming a medical mobile unit at Venice, Louisiana. Local animal welfare activists began reporting that dead birds and other marine life washing up on beaches were being secretly removed at night by anonymous officials, evidently to hide visible evidence of the impact on wildlife.
In the early days of the catastrophe, deeply disturbing stories were told by fishermen who reported finding vast, floating graveyards of dead birds, dolphins, and whales near the Macondo site; these were secretly burned at night. Some argued that this was the reason for the media aerial blackout over the “Ground Zero” site in June. Huge smoke plumes suggested that these vast, graveyard-gyres of dead marine life were being burned in secret out at sea. Later, images of half-burned whales and turtles began appearing on the Internet, exposed by Greenpeace and other organizations.
There were eventually media flyovers of the "Ground Zero" site... in planes piloted by the Coast Guard, presumably in areas the Coast Guard wanted to go to. The article discusses this:
By the time we got to the Deep Delta, BP and the Coast Guard had arranged carefully choreographed boat and aerial tours for certain accredited media. How carefully choreographed these tours were, became instantly clear when we were taken by boat, accompanied by a local scientist, to an island to look for oil-damaged pelicans. I instantly knew the island was not a pelican rookery, for there were no mangroves, and the feral pig scat I saw on the beach told me that no pelicans would be nesting there.
The journalist also claims:
The Louisiana ACLU wrote an open letter assuring the media that the Coast Guard ruling was unconstitutional.
This is true, and the letter can be found in the Wayback Machine, although there is very little mention of it in the press!
Local newspapers carried reports, as did bayou bloggers and alternative Internet media outlets. Then the Coast Guard did another abrupt about-turn, assuring members of the media they could get unfettered access to oil-damaged areas, but only if they contacted the Houma Joint Information Command with certified press credentials. This, too, however, proved extremely difficult. Karin and I managed to get on a Coast Guard media-flight to the Macondo site with six international journalists only by calling the Houma JIC in the middle of the night, eventually managing to talk the bored JIC official into giving us press passes.
So on July 18, we flew in the Coast Guard plane out to the Macondo site. Only after the flight, however, when I traced our route on a map later that evening, did I realize how closely choreographed the flight had been. As we flew fifty miles out to sea, the Coast Guard carefully took us over the areas of marsh and ocean that were not excessively oil-soaked.
This media coverup has been analyzed in an academic journal.
An initial report of censorship on 20 May occurred when a crew from CBS
News attempting to document the spill was threatened with arrest if they did not
turn their boat around. The Coast Guard official reportedly explained, “... this is
BP’s rules, not ours” (Edwards 2010a).
Mac McClelland, a journalist for Mother Jones, also was confronted by police
when trying to document the spill. On 22 May 2010, McClelland was refused
access to Elmer’s Island by a Jefferson Parish Sheriff deputy who claims he is
just “doing what they told me to do”. While trying to gain access to the island,
McClelland (2010a) recounts her exchange with BP representative Barbara Martin,
who makes clear that BP and the Sheriff’s office have a “very strong relationship”.
When McClelland (2010a) inquires why she cannot see Elmer’s island, Martin
responds that “... BP’s in charge because ‘it’s BP’s oil’”.
One citizen, Drew Wheelan, the conservation coordinator for the American
Birding Association, had been stopped by BP private security while filming the BP
building/Deepwater Horizon command centre in Houma Louisiana from across
the street in a field, on property not owned by BP. He was then approached by a Louisiana Sheriff’s deputy who asked for his identification, stating that “BP didn’t
want people filming.” After being allowed to leave, Wheelan was then pulled
over and questioned by BP Chief of Security as the officer stood by. Once they
did finally let him go, he was followed in two unmarked security vehicles for an
additional 20 miles (McClelland 2010b).
The NYT article affirms that the Department of Homeland Security was involved in the coverup.
“They said it was the Department of Homeland Security’s response-wide policy not to allow elected officials and media on the same ‘federal asset,’ ” said Bryan Gulley, a spokesman for the senator. “No further elaboration” was given, Mr. Gulley added.
I find no evidence that Obama gave a specific, public order about this, or that DHS organized the whole thing. However, this is not to discount the OP's paranoia, as there was DHS participation, and according to the academic article, there does not seem to have been any investigation about who organized the coverup:
Within the Coast Guard, however, pressure from above had prohibited the
agency’s release of public information that had not first been reviewed and
approved by the White House and DHS. The restrictions on media access to the
Gulf of Mexico and response operations thus appear to have come directly from
the Obama administration itself.
Overall, this is a mixture of true, false, and unverifiable claims.