Deal Breaker
I think a misunderstanding of what a Dealbreaker is. It does not mean, someone who literally breaks deals. A dealbreaker is, in common terms something that is absolutelly neccessary for a deal to happen, if it is not there then the deal doesn't happen.
Example: I want to buy a horse and you want to sell me one. However I only have $100.00 for both horse and saddle. If you are not willing to throw in a saddle along with the horse, then the price of the saddle becomes a dealbreaker. Without it, there is no deal.
So a person who is a Proven Dealbreaker, is a diplomat who's skills are so good that they are needed, otherwise the deal would probably fall through.
So, a Proven Dealbreaker, is a good name for +2 diplomacy. Its just a misunderstanding as to what a dealbreaker is.