1)You say this 'amendment' comes out of nowhere. If it was an amendment and you haven't signed it, is it valid?
2)A shall not indirectly contact C. But you have already indirectly contacted C through B. As B was aware of this, I'd think this should be void. I would also think that you getting an @C e-mail address kinda shows that you have already been in contact with C, assuming C doesn't give e-mail addresses to people with no connection to C.
3)If C is a start-up with no value, and B is nothing, why are you even worrying about this? go out and get some business without B or C - I'm assuming your contract doesn't say that you have to work exclusively with B. If it does, than I wouldn't say B has nothing, as they have you!
From what you describe, it does not sound like it's worth pursuing or getting worked up over.