I don't know if I buy into Missy not being who she says she is. It would work just fine within the internal logic of the show, but from an exterior perspective I don't really see it happening. It would be a bit of a disappointing red herring for her to turn out not to be the Master after that reveal, and you have to be pretty careful when stringing your audience along with stuff like that. There's huge potential for it to deflate a lot of the engagement in the story, and I think Moffat knows his stuff enough to avoid that if possible. Unless of course he has an even more fun twist waiting next week, although it would be hard to one-up the return of a female Master!
The drumbeats were very much one of those RTD-canon things that had a big impact on the ongoing lore of the show, and which Moffat has been gently extricating ever since he took over. He's been making a clear effort to return the show to neutral ground, where new writers won't have to deal with so much prescriptive canon moving forward.
Things like the Doctor being this mythical figure known throughout time and space for his exploits, the Daleks having to come back from the brink of extinction time and again, Gallifrey/Time Lords being dead and gone etc. are all good examples. Also the idea of the Doctor having a family (and his sudden interest in romance in general during RTDs run) seems to have been quietly swept under the carpet.
So yeah, it wouldn't surprise me if the drums have been put to bed as well. They always struck me as a pretty lame and generic reason for the Master to be a bad guy. He's much more fun and interesting when he's just being evil and crazy because he loves the heck out of it.