Does this actually work to coock it?

Doesn’t seem safe to try to cock it while it’s in the hot spring. Maybe yoy could find a way to make it could actually work…but why?

Is it at all edible?

Sous vide method would probably have the most chance Of being edible since the turkey would be vacuum sealed

Is there any environmental impact or downside?

Yes. That’s why the park service is saying not to do this. You’d be introducing new chemicals into a delicate ecosystem and also potentially physically damaging it.