The coordinates Missy gives for the location of Gallifrey are only half a lie. That location (10-0-11-0-0x02, as established by The Doctor in Pyramids of Mars) is where Gallifrey used to be, before the Time War and more to the point before the Doctor's trick to "hide" it. Missy's lie is that it's emerged from its pocket and returned to its original place, but the coordinates were plausible.
As for Missy not really being The Master, it can't really be ruled out, given that not only are Time Lords notoriously fickle with the truth, but so Is the showrunner! But there's little evidence for it. Everything about her plot in "Dark Water/Death in Heaven" fits with their longstanding relationship with the Doctor, and with the characterization of the Master as truly unhinged. The Rani, for example, was always a fairly calm and rational adversary, more amoral than immoral. No other Time Lord the Doctor's crossed paths with has ever been quite the same kind of murderously bananas, and none of them know the Doctor so intimately as to know exactly how to hit him for maximum impact!