Because Oda has already planted the seeds for Law to be in Elbaf. Law knows of the scarred man, Big Mom claimed she will get her revenge on both Kid and Law, Law wants to learn about the Will of D.
That doesn't say anything in regard to Law's involvement in Elbaf. Law's learning about the Will of D one way or another. We know for a fact that he's not going to be the one to reach Laughtale—not on his own at least. The PK contender b/w Law and BB is BB, not Law.
if he kills Garp that's better fuel to the fire
Sure, but how will it happen though? If Garp doesn't come to Winner Island, the chances of Law escaping reduces—it's not impossible but it's hard. Even if he escapes, he's probably losing his crew to BB. Another possibility is his getting captured.
You think Law would just take the L and quit? He would probably device a plan to steal his glyphs back. That's where Kid and Luffy come in.
I'm thinking more that BB retreats when he realizes there's a marine fleet attacking his HQ's
Possible, but I don't think Law will run back to Luffy asking him to help him get back his RP copies from BB. Law's chances of attacking Luffy or Kid to get their RP copies are higher than his asking them to help IMO. If Law gets captured by BB, then there's a good chance he too gets involved in BB fight and then travels with the SHs to the final island.