If you do the amputation, note that if you leave the joint end, the joint will still produce joint fluid which will fill the area. It may require draining.
In our case, when we had my turkey's wing amputated (after a clean dislocation), the vet amputated mid-bone which did cause air leakage, but the air leakage healed and ceased. Of course, he was a vet with the right equipment.
I understand that there are 'pinion' surgeries which (I believe?) still leave the joint end.
In any case, bird bruises can be green - and broken bones can cause leakage. Maybe reading this post can help:
https://www.backyardchickens.com/forum/viewtopic.php?id=211237
If there is indeed gangrene, then yes - it does have to go. And yes - if there are any chances of maggots, the maggots MUST go. People constantly throw out the "but maggots clean wounds" theory. But this is not so in a real life situation. In real life, maggots spread disease through a wound. We're not talking about maggots cultured in a sterile lab and applied by a trained technician, or even the type of maggots they use there. In real life, blow flies will eat live flesh and burrow into the insides of the bird and kill them. Regular maggots spread bacteria. So thank you for sharing that information.