Yes, it is possible some viable mutants will result. However, would the chances be any higher of producing a strain of E. coli that are deadlier to humans? I doubt it.
Increasing the mutation rate has to increase the chances of mutants that are both viable and more harmful. Mutations from radiation are essentially random, so more mutation events means more mutation varieties, and so a greater chance of something very bad happening. Analogy: a handful of 5 6-sided dice. If you roll those dice once an hour, your chances of rolling all sixes in any day is very low: 1 in 324. If you roll them every 5 seconds you don't have a better shot at all sixes in any particular roll (1:7776) but your odds of hitting all sixes at least once in a day are 720 times higher, and most days you'd hit it twice. And have serious hand cramps. :)
It should also be noted that most E. coli strains are harmless to humans, and our guts are full of them. It is evolutionarily problematic for a parasitic microorganism to kill its hosts, so most things that take advantage of living inside us do so without doing damage. We actually are dependent on our gut flora (including E. coli) for healthy digestion and absorption. The deadly strain of E. coli is believed to be the result of fairly recent (decades) incorporation of Shigella virus toxin genes by E. coli in cattle feedlot environments where shortening host life is essentially meaningless but triggering large volumes of waste helps the bacteria spread from its already-doomed (i.e. future hamburger) hosts to nearby others with a few more days of life.
Back on topic, is the uranium-phosphate that is produced still radioactive, or does this just make it easier to extract and remove from the environment?
There is no (bio)chemical way to eliminate radioactivity. This is just a way to isolate Uranium out of whatever soup of other stuff it happens to be in. Isolating the Uranium out of waste is useful because the Uranium is both hazardous and potentially reusable, but it only deals with part of the problem. Nuclear waste comes in many different forms, and the forms that would be targeted by this sort of tactic also are full of Uranium's decay products, many of which are themselves also radioactive and/or toxic. Absent a truly radical revolution in our understanding of nuclear physics, there is no total solution to the problem of waste from fission reactors other than time. There's nothing we can do to prevent radioactive decay and only a few narrow cases (i.e. fission reactors and bombs) where we have figured out ways to speed it up, so the "solutions" are all aimed at trying to concentrate and fractionate the various forms of bad stuff in the waste and find ways to safely store or reuse each fraction.