Hello,
First of all, I'm no near to be a doctor or someone who has had the surgery done.
But I have a decent amount of knowledge about LL so,
First of all, "amputation" is nearly impossible and I've never seen such a case like that. But I'm not going to answer that question since maybe there are some parts that i'm missing. Yet I can approve that amputation is the least likely thing to happen during the procedure.
Secondly, I guess you cannot know if you have non-union before having the surgery as far as I remember, it was asked before. And people said that you never know unless you get the surgery done. So, I guess you gotta be lucky.
edit: For personal and insanely reasonable reasons, I never recommend going for more than 2,7 iches on femurs.
Best Regards