What are the alternative treatment for possible chronic bone infection in a below knee amputation?

Antibiotics. Antibiotics are the main treatment option. If these don't work opening the area surgically and cleaning it may be necessary. In severe cases the amputation may need to be redone at a higher level.
Depends. Few options really. In acute om, may be able to salvage with hbo and long term IV antibiotics. However, in general, if the bone is infected, it is highly likely that that part of the bone will need to be resected or the bka moved to an aka.
Hyperbaric Oxygen. There have been some reports of hyperbaric oxyegen chamber as being beneficial for lower extremity wounds. This would fall into the "alternative treatment" category for osteomyelitis (bone infections).