That was not BSG's premise but the resolution of the arc story, that has nothing to do with premise, which is what we are discussing here.
I don't know what your definition of "premise" is - maybe BSG's premise was something simplistic, like escape the Cylons and find Earth - but Cavil was the character who impelled the action of the plot and therefore is key to the whole frakkin' thing.
Cavil turned out to be the most important character in the story. And yeah, he was shoehorned in long after the story began, which is what caused the clumsiness of the story - writers really shouldn't start a story without taking into consideration their most important character, even if they don't plan to introduce him till later.
The primary mistake in this forum is fans come in, post their wacky premises and state nothing about how they will make it an exciting weekly show.
Well, that's certainly true. You could take any wacky premise and make it boring or make it great. The elements of good storytelling are basic: characters we care about, cast of diverse enough types that they play well off each other, plotlines that stem from the characters' unique personas and are not just pulled out of the writers' asses (especially not by making characters stupid, forgetful or insane for the convenience of the plot - the sure mark of an inept writer), themes that matter, plot twists that are neither too predictable nor absurdly out of left field, and let's not forget the spacebattles, splosions and sex.
Any premise - time travel, spaceships manned by Tholians, the Federation falling or having a civil war, even the dreaded Starfleet Academy - could be terrific if you get the frakkin basics right.