Yes, umami exists as a taste and is distinct.
Triggering mechanisms have been identified (http://www.pnas.org/content/99/7/4692.short):
human T1R1/T1R3 responds to the umami taste stimulus L-glutamate, and this response is enhanced by 5′-ribonucleotides, a hallmark of umami taste.
It is listed without controversy in peer-reviewed work (http://www.sciencedirect.com/science/article/pii/S0092867403008444):
Sweet and umami (the taste of monosodium glutamate) are the main attractive taste modalities in humans.
The sense of taste is responsible for detecting and responding to sweet, bitter, sour, salty, and umami (amino acid) stimuli.
It is distinct from the other tastes (same ref):
In humans, monosodium L-glutamate (MSG) and L-aspartate, but not other amino acids, elicit a distinctive savory taste sensation called umami.
The theory that it is a distinct taste has testable predictions that have checked out (same ref):
If T1R1 combines with T1R3 (T1R1+3) to generate the mammalian umami receptor, then a knockout of T1R1 should also eliminate all umami responses.
Here's more recent work (2009) saying the same thing (http://ajcn.nutrition.org/content/90/3/723S.short):
The uniqueness of umami perception is based on several lines of evidence. First, numerous perceptual studies have shown that the sensation aroused by MSG is distinct from that of the other 4 taste qualities. Second, biochemical studies that show the synergy of the binding of MSG and 5′-guanylate to tongue taste tissue mirror this hallmark perceptual effect. Third, several specific receptors that may mediate umami taste have recently been identified.
Taste vs. flavour
There is a difference between the two: "flavour is the sensory impression of a food or other substance, and is determined mainly by the chemical senses of taste and smell". So, change the taste; change the flavour. If there exists a umami taste, then it can be used to change the flavour.