Your question is a bit complex: "Does the use of Roundup accumulate in mammals and cause illness?"
I'm going to focus on a slightly simplified version of the question, for sanity sake:
Does glyphosate cause illness?
The answer to this much simpler question is clearly "Yes." But that alone does not mean that glyphosate is pure evil; many things can cause illness, depending on concentration and other circumstances. See my closing comments for more on this...
The Pesticide Action Network of the UK has a well-referenced, and seemingly un-biased fact sheet on the topic of glyphosate. It discusses varoius types of toxicity, and on the topic of mamalian toxicity, it says in summary
There is nevertheless evidence of toxic effects on humans as well as environmental toxicity, indirect environmental damage and resistance in some target weed species.
The acute toxicity of glyphosate itself is very low. According to the World Health Organisation, the oral LD50 in the rat of pure glyphosate is 4,230 mg/kg, or 5,600 mg/kg according to Monsanto(6). The low acute toxicity of glyphosate can be attributed to its biochemical mode of action on a metabolic pathway in plants (called the shikimic acid pathway) which does not exist in animals(7). However, glyphosate can also disrupt functions of enzymes in animals. In rats it was found to decrease the activity of some detoxification enzymes when injected into the abdomen(8). In general, controlled toxicity tests report adverse symptoms from exposure to glyphosate only at extremely high doses, ie several grammes per kg body weight.
While glyphosate itself may be relatively harmless, some of the products with which it is formulated have a rather less benign reputation.
Some literature suggests that glyphosate can cause some chronic health effects and birth defects in certain test animals when administered at high doses over prolonged periods(15). Chronic feeding studies have shown reduced weight gain, blood and pancreatic effects, but no evidence of carcinogenicity to humans. A US EPA report says: "Effects on pregnant mothers and foetuses included diarrhoea, decreased weight gain, nasal discharge and death of mothers and kidney and digestive disorders in rat pups"(16).
It is extremely unlikely that human users or members of the public would be exposed to doses as high as those used in the trials, but extrapolating toxicity data from rats, mice and rabbits on which trials are run, to humans can be inaccurate and misleading.
Note that this answer does not address the entirety of the question; this only focuses on the direct toxic effect of glyphosate on mammals; the question also makes the claim that glyphosate can destroy gut flora; this claim is not addressed, and that glyphosate can accumulate in bones, thus "hindering cellular detoxification" (whatever that means).
I think those other questions could be answered (the one about gut flora in particular), but I believe this answer shows sufficiently that glyphosate does have an effect on mammalian health. If all you want is a "yes or no" answer, the answer is clearly "yes." If you want an answer about degrees to which glyphosate harms mammals, the answer is much more complicated, and then the question of gut flora, etc, may be more relevant.