Do you think Littlefinger will have to try and get Sansa an annulement in order for her to marry Harry if Tyrion doesn’t turn up dead soon? If so how would he pull that off?

Good question! I’m not sure. It could be that he’ll get a septon to declare that Sansa didn’t freely consent to the marriage with Tyrion and therefore it wasn’t valid, it could be they’ll go with an Old Gods/New Gods thing, or could be some other dodge.