"Nor has it been explained what "missy" has to do with Clara being the impossible girl. It is explained why the Doctor and the current Clara were destined to meet - but not what that has to do with the previous 2 Clara's......"
Ah now, Missy had nothing to do with Clara's later jaunt into the Doctor's time stream. That was something that Clara herself decided to do on the spur of the moment in "Name of the Doctor". Whoever was the companion when they arrived on Trenzalore probably would have done the same and ended up split into the time stream. That was the reason for her appearing in the previous season, on the Dalek Asylum and Victorian London - they were two of her "rescue missions". It's a standard case of "timey-wimeyness" where cause and effect happen in the wrong order. Somebody does something in the future ("Name of the Doctor") and it affects the past ("Asylum" and "Snowmen"). Missy had nothing to do with that, and had no idea it would happen...and it didn't affect her plans anyway.