Indeed, if you take the pieces one by one as you did explain why Law approaches to Luffy, an at this point it's clear. But as you say the "D" matter was what Cora-san introduced in Law's head (and it kept growing more and more with the fact that he sacrificed his life for him),
Luffy won't pay much attention to the "D" unless he'd see his father and somehow he'd take some care about it, Law is smart maybe at some point he will gather with the revolutionaries and finally they explain him. Just speculating we don't know what Luffy learned or know about the "D" I guess nothing.
BTW nice one, remarkable conclusion.