NO REALLY SPOILERS
So it is just imperative that I say, there is no way Tasha Lem is not River Song. There's no way there's just some person the Doctor hasn't seen in hundreds of years who he has flirty banter with, initiates making out with, who he talks to about "fighting her inner psychopath her whole life", and oh who just happens to be able to FLY THE TARDIS, who is not River Song. I'm not saying chronologically or any other kind of logically it makes sense, but when has that ever stopped anything.
Actually there are two ways she could not be River. 1. Moffat is INTENTIONALLY SCREWING WITH US AND RUBBING IN HIS RIDICULOUS ANTICLIMACTIC DISMISSAL OF RIVER. 2. He has become so incredibly lazy that he wanted her in the episode but, having already killed her, just made up someone else to be her. And have her abilities. And her relationship with the Doctor.
So yeah, basically, she is River.
Edit: I am adding Jay's revelation that the Papal Mainframe is a big computer. A big. Computer.