I find your "possible scenario" to be very frightening. Personally I would never trust that dog's recall. She is bred to follow her nose and much as you try to train and even seem to achieve success I would never trust her in a situation that might be dangerous to her.
You could use a martingale collar on her, they are very hard to slip out of. I think they would have to break before a dog could get out of one.
Our friend who runs coonhounds puts a gps locator collar on his dog. Now, he intends the dog to run because he hunts her but the gps locator allows him to find her. An idea to consider?
Two neighbours have rescue hounds, a Beagle and a Bassett. Neither will let their dog off leash.
If you want to really try though, is there a hound club you could contact? I agree with your thinking that the tracking class might give your dog work and exercise she enjoys but it does not train for recall. Though a nosework class I attended does use the dogs off leash and they stay nearby because the scents they are trained on aren't usually found leading them away, but I think that varies.