In day-to-day use, you would not hear people saying "brother", instead. It just doesn't happen. "Bro", in this context, has evolved.
No, but it would be unusual. You don't hear people using a term like "crypto brothers", for instance. "Bro" carries a different connotation, and it isn't a positive one in this example.
I don't think it is simply an abbreviation in this case.