I am so sorry you are going through this. As a cat lover I filled up just reading your story. Like the others, I have a little tripod named Scully who I adopted from a family who did not want her anymore. Due to circumstances with that family I do not know the true story of how she lost her leg (right, back leg) but her vet says it definitely happened after birth (as opposed to an accident later in life for example) either way, I too just wanted to say how well she gets along. In fact she chases and plays with my larger polydactyl Willow all the time! She can jump up on the bed with just one back leg, no problem.
best of luck with everything.