I didn't say it was more likely, but it's still likely. Consider how many turtles actually make it passed the first couple years. This one is still small enough to have many predators to worry about. That's the only thing the op should have to worry about. The closed environment is an overly used point, in fact I would go as far as to say the chances are 0 that the turtle will catch something in an aquarium that it wouldn't have already been exposed to in nature. There are far more diseases out there in the wild, than there are lingering around in an aquarium with only a single occupant. Consider that these are scavengers, and will eat dead rotting animals and decaying plants. If it was anything to worry about, licensed rescue groups, aquariums, zoos, fish and wildlife wouldn't head start, and release 10s of thousands of hatchling turtles into the wild every year.
I suppose if someone were a professional zoologist and raised animals with the goal of release just as these scientists you speak of do then there would be nothing to worry about. I'm glad that you are so confident in your assessment of ecological threats. I know I'm not qualified to make such judgments.
If you really want to release him into the wild, contact a park ranger. I know in my area they have a "raise to release" program and, providing you prove the turtle can fend for him/herself, you can release the turtle in an area that the rangers recommend.