1. The show is not as beholden to the books as it once was, and this includes in-world logic. While Sansa is still married to Tyrion, the marriage was never consummated. While we know a septon usually needs to nullify it, it's not exactly like we will have Tyrion riding along to object, as he's a wanted kinslayer, and Cersei, while she will certainly find out and object, is not going to send an army north to wrest her back from the Boltons. The show established this when Roose pointed out that the South wouldn't send an army that far North to help against Stannis, as they have never done in show canon history according to Roose. The crown would simply end its allegiance with the Bolton's, but what Roose seems to need now is the North's complete allegiance, which Sansa will help with, not the crown's (In show logic, not book :dunno: ).
To have this make any kind of sense, the show had to change the rules. It appears that Tywin Lannister's death somehow destroyed most of the Lannister power, according to Littlefinger, as the name is not what it "once was".
My ultimate point is that show logic and world consistency is not intrinsically tied to book logic and world consistency as it once was. Give out about it all you want, but I wouldn't fall into the trap of "that can't happen cos x"; the rules have changed, and will keep changing, and tbh, I can understand why they have altered some of the plot threads and consistency for this season. Having Sansa join Winterfell storyline has her story merge with a story the viewers are already familiar with. It also means not have to cast a whole new Vale story, and Sansa can interact with key, familiar characters.
Even though some of the inconsistencies irk me, I'm not as upset about them as I'd thought I'd be. The sansa winterfell merge is actually quite interesting, and for once I feel like the stories are able to breathe again, without having to fit too much in. I've accepted the inconsistencies as the inevitable ripple effect of plot changes, running out of source material for various characters, and budgetary and casting concerns. It's bad writing, but the show oddly enough still has me hooked.