I read an excellent fic last night with that as the hypothesis: that everything, including Mycroft giving Moriarty information, was planned by the brothers. What Moriarty did with the information was a surprise, hence the need to include Molly, but the need for Sherlock to die was a contingency they had covered. It made a lot of sense to me: I just can't believe that Mycroft could be used that easily, or that he'd sacrifice Sherlock with so little gain - not after he bent over backwards to get Sherlock out of trouble in ASiB.