1. Re: the Craster's expedition.
This was done largely to provide an action sequence at the mid-point. It's due to pacing and plotting, and the fact that the third book crams a TON of action in at the end of it. Several battles, duels, and other momentous events all get dealt with back-to-back-to-back in rapid-fire succession. But the problem is that the setup for it takes a bit.
Character-wise, I think it does do one bit where it really showcases Bran's determination to head north, and his growing powers. Right here, he has a chance to go be safe with Jon, to give up his quest, which -- let's not forget -- is based on a vague dream and Jojen's interpretations and greensight. It's sort of a "Frodo" moment where he has to decide "Do I pack it in, or keep going?" A younger Bran, the one who moped in bed after being crippled, the one who sulked at Winterfell while bored with menial tasks of ruling, that Bran would've gone back to the wall with Jon. This Bran, however, has a mission and is committed to it.
Plot-wise, the only thing I think this does is stretch out the season a bit until we get to a few other Jon moments. Plus, Jon needs to be recently wounded and not at 100% for the upcoming bits, but time had passed too far in the season already, what with him showing up at Castle Black at the very end of last season. So, this provides another opportunity to do that.
2. Re: Dany.
I have NO idea what they're gonna do to fill the time with her while she sits. As for the people saying "Oh man! She's gonna go back to Westeros now!!!" hahahahahahahahahahahahaha That's cute.
3. Re: Cersei.
I think her discussions with Maergery, Tywin, and Oberon served two purposes. I think she genuinely wanted to have people supporting her family and her kids, so that part is legit. But she also clearly wants to influence the trial without saying "So....you're gonna vote to behead Tyrion, right? 'Cause he needs to die. I mean, he REALLY needs to die. Like, yesterday. K?"
4. Re: future seasons.
Prior to having read them, I assumed that they'd do Books 4 and 5 chronologically over appx. 4 seasons. Having read Book 4...I don't think there's enough for 2-seasons worth of Book 4 material. TONS of Book 4 is description and internal monologue, much of which will play out in behavior, meaningful looks, etc. and otherwise be turned into 2 seconds of acting rather than 5 pages of narrative. I think they'll end up creating a lot more scenes designed to showcase the
types of things the books do, if not necessarily to show exactly what the books did. Some things will obviously change, like Brienne's travels. Since many of the foes she faces on that journey simply don't appear in the show, they'll have to create new characters to fill the void, like how they created Locke to fill the role of Vargo "Thapphireth" Hoat.