X2, I have not read the context of your questions but here is what I know
Amputation is not just allowed, its been done. There is a rivaayat that one sahabi asked his leg to be amputated while he is reading namaz so he cant feel the pain.
Amputation is usually done when the limb is infected so infected limb is no good for anyone, anyway.
Amputated limb should be wrapped and buried in a Muslim graveyard if possible.