Well others have suggested that this deviation from the books was bad writing cuz it made no sense. That's what I thought you meant when you said it was a bad plan.
I agree wholeheartedly it won't work. Bolton won't win. He can't win. He sealed his fate at the Red Wedding. But what would be a better plan for Bolton? You suggested he should wait for a "flawless" plan, what could be a better plan than gaining an allegiance with the only living Stark?
Oh no doubt no doubt. I can see the disconnect.
I didn't mean that the Boltons should wait for a flawless plan. I meant the rest of the northern clans should. They're probably going to have one shot to attack Winterfell and free Sansa.