I've never heard of a turtle eating poison ivy, that's really interesting. I have heard of them eating nettles, though, and that can't be very tastey.
As far as missing legs go, there was a poster here once who had a female turtle (I can't remember species) that successfully nested while having only two nubs for rear feet. It apparently took all night and into the morning, but her half legs were enough to dig in a softened area and lay eggs.
I myself have a male spotted turtle that is missing one leg. He mates just fine with the female, and chases her everywhere. If there were competition, perhaps he would have less luck, but if he has the female to himself, which a turtle could have in a situation in the wild, then I doubt it would stop him.
Are these rear legs or front legs it is missing?
I heard of one keeper with a one legged painted turtle that did just fine in captivity. It climbed onto it's sloped basking spot, ate, swam, and otherwise seemed pretty normal.
Anyway, I wouldn't discount a turtle for missing legs. Especially a wood turtle. Those are some tenacious and hardy suckers there.
Also, on the mites and ticks... once the turtle goes into the water for any length of time, they will be gone if they ever get onto it. Then the problem becomes leeches... poor guys can't win in the wild. They manage to survive, though.