Nevertheless, the vet said today that the best solution is, as I predicted, to amputate the leg at the hip. I think she'll be much better off. I'm just glad he didn't tell me to kill her and have done with it. He was a bit cold when we went into the examination room, but he warmed up and is even giving me a "deal" on the price of the surgery. The deal is still rather high, but I took on the responsibility and I'll see it through. He did offer a solution to get the spading paid for by animal protection association that is based in France.
Oh, and the kitten that I estimated at four months is actually an adult or close to it. She weighs between 4-600g and will probably stay a miniature cat all her life. That might help get her adopted, but the vet was not very encouraging on that subject. We expected it anyhow, and he didn't tell us anything we didn't already know.
I was trying to think of a solution and I might have hit on one if I can make it work. I jokingly asked kmkibble75 if he wanted to adopt her, but what is that is the way to go about it? Maybe I can find someone in France who would be willing to save her. J's mom is supposed to come in April or May, and the cat should be healed, spade, tatooed, and hopefully a bit cuter before then, so maybe we can find her a family to go to and J's mom can take her back on the plane.