I think it's believable, Darth Plagueis was powerful and he was simply killed in his sleep like a chump. Palpatine was really powerful and Vader just powerlifted him and threw him over a railing.
Which is kind of hilarious to be honest.
Those dark side users are always overconfident and like smelling their own bullshit but then get stabbed in the back and never even saw it coming. It's a tradition.
As far as backstory goes no doubt the EU will fill in all the blanks for Snoke. One could argue that should have been in the movie but the question is what purpose it would have served the story? It would have just been there to satisfy fans' thirst for lore but it's irrelevant to the immediate story and to the characters and their arcs.
Palpatine had no backstory given in the original trilogy either. We didn't even know his name, the words "Sith Lord" were never even mentioned.
All that were details left for a time when it actually became relevant to the story told (that is, the prequels).