GRRM literally says he's had conferences with them, numerous ones to make sure they are in sync with his view of major characters. That means he has creative rights (not legally, but practically). He also says he doesn't think it ends in a way that's different to his in outcome (note he would have certainly been one of the few to see the final scripts here).
Just stop arguing with the end bits now, GRRM has pretty much said he would have ended it in the same way with major characters.
The finale was 1hr 20 long, to wrap it all up in that time.. Its the best it could have been. Ideally they needed to extend the whole show by a couple seasons but they couldn't.
Seriously laughable you're still trying to claim GRRM had no creative input into where the show goes when he's literally filmed saying the opposite