is it safe to assume that Beric Dondarion’s role in the north is entirely a show invention, or is there any reason within the text to believe that he’s coming back post-stoneheart?

The former. Beric is dead for good in the books, and not coming back. 

Here’s why I think it’s safe to assume this: 

  • in the post-episode bits and interviews about the show, the showrunners have made a big deal about needing the dragon to die then working backwards from there and hitting on the rescue mission. 
  • as we’ve seen with Craster’s Keep and Hardhome (and sort of the Battle of the Bastards, although that’s more a mix of this phenomenon with the Battle of Ice from the books), the showrunners really like doing action scenes with Kit Harrington and will invent stuff out of whole cloth to make that happen.
  • the Wight Hunt had a lot of similarities to some of those invented scenes:
    • they involve a bunch of named characters in a situation that would normally involve a huge body count, but only redshirts die (except for Thoros) so there’s no real impact to the plot, as far as characters not being alive who they need to do stuff later. 
    • they are often circular in terms of motion: Jon goes to Craster’s Keep and then comes back to the Wall, Jon goes to Hardhome and comes and then comes back to the Wall, and now Jon goes to near Eastwatch and then comes back to the Wall. This way, rather than leaving our protagonist in a new location as their status quo (like Dany capturing Meereen), the main characters are back where they need to be to do the rest of the plot. 
    • they often involve some impressive spectacle in terms of CGI: Hardhome gave us lots of wights, this gave us lots of wights plus dragons, so it’s an escalation from Season 5.
  • It also involves a lot of characters who are unlikely to be in the same place at the same time in the books. Sandor is at peace on the Quiet Isle, Beric’s dead for good this time, Thoros and Gendry are with Lady Stoneheart whose plot was given to Arya, Tormund is at the Wall, Jon’s dead, Jorah is still in Essos. It would be very awkward indeed to get them to the Wall together while everything else is going on in time for this mission.
  • Finally, the whole get-a-wight-to-convince-Cersei thing won’t work with how the books handle the wight rules. In the books, wights stop reanimating when they get south of the Wall and rot away into nothingness. While the show dropped this storyline in Season 2, in the books Alliser Thorne is sent on a mission to convince the Iron Throne of the threat to the Wall and fails. 

Why do you think that destroying the Sept of Baelor would be worse than the Red Wedding? What would the consequences of destroying the Sept in Westeros?

Because for anyone who believes in the Seven, and indeed the majority of Westerosi do, the Great Sept of Baelor has been the center of the Faith for ~150 years. It’s associated with the closest thing the Faith has to a saint, Baelor the Blessed, who is known to the smallfolk as Baelor the Beloved. 

The Red Wedding is an attack on men, albeit one that violates the laws of both gods and men. The destruction of the Great Sept would be an attack on the gods themselves, the most impious action imaginable. And everyone inside is going to be a holy martyr.

As for the consequences, I think they’d be absolutely lethal to the legitimacy of the regime. You’re probably going to see a new revolt of the Faithful, as all of the Poor Fellows and Warrior’s Sons who survive the explosion (and keep in mind, there’s a lot of them out in the Riverlands atm) rise up, probably spearheaded by Bonifer Hasty’s Holy Hundred in command of Harrenhal. And you’ll likely see substantial defections to any alternative monarch. 

Jon being King of the North: on what basis could he claim the throne? The North isn’t overthrowing monarchy/aristocracy, after all, they still all want their castles and titles, they just want to ignore Ned’s legitimate daughter in favor of his illegitimate son. What happens when it comes out he’s a Targ?

Jon is King in the North on the same grounds that Robb became King in the North – through right of acclamation as opposed to right of inheritance (since Robb’s father was not King in the North before him). It should be noted, moreover, that King in the North != Lord of Winterfell. 

As for him being illegitimate, Lady Mormont spoke for the majority: “I don’t care if he’s a bastard. Ned Stark’s blood runs through his veins.”

Him being a Targ will be a complicating factor, but when the Targaryens land on Westeros with a giant army and three dragons, having a Targ in your corner might not be a bad thing. 

Still about Cersei taking the Iron Throne in the show. Thinking more in terms of short term claims: as of now nobody really knows Cersei blew up the wildfire caches. With the most likely successors all out of the game (including Cersei’s own children) won’t she be the most likely ruler anyhow, anyway, until other clamaints even realize they do have a claim that may precede her? I’m guessing she believes she has time/is able/ to organize to counter those, somehow…

The King, the Queen, the Queen’s brother and father, and the High Sparrow all died on the same day and then Cersei became Queen and declared war on her former in-laws. I think people are going to put two and two together. 

Could you explain the proximity arguments for the Estermonts and Florents, please? Why does marrying a family member into the royal family give you a claim on the throne? And if the Estermonts and Florents have a claim through proximity (presumably from their marriages to Steffon and Stannis), shouldn’t Cersei also have a (better) claim through her marriage to Robert?

So here’s my way of thinking, and there’s no reason necessarily to think it’s better than anyone else’s (thinking of @nobodysuspectsthebutterfly, @goodqueenaly, and @warsofasoiaf here):

If what matters is who the closest blood kin of the king is, then the Florents are kin to Princess Shireen and inlaws to Stannis (if you accept that Stannis and Shireen were the lawful kings/heirs to the Iron Throne). Next most recently, the Estermonts are kin to Robert, Stannis, and Renly through Cassana Estermont, regardless of how you see the succession going from that point on. After that, the Targaryens are kin to Robert, Stannis, Renly through Rhaelle Targaryen,  as well as to all previous kings. 

Cersei’s blood relationship to Robert Baratheon is much more distant than any of those, going back 90 years Gowen Baratheon and Tya Lannister, but even then that pairing died without successful issue, and 120 years ago when an unnamed Baratheon woman married a male Lannister and had issue. It’s highly unclear whether Cersei has any blood connection to Robert.

Now, there is an argument that she’s the mother of Joffrey and Tommen, but…A. their parentage and kingship was publicly put in question and Cersei has already admitted to incest in one case, and B. claims rarely go up the generations in that way, and C. even if the Lannister branch was chosen, Targaryen and Great Council precedent would suggest that a male cousin rather than Cersei herself, would inherit. 

So is Rickon really just a shaggy dog story in the books given his fate in the show? I’ve always felt that the Davos-goes-to-Skagos-and-retrieves-Rickon plan seems too convenient to work as intended. Wyman Manderly is going to die (either from his recent wounds or being killed by Stannis before he can reveal his true colors). And doesn’t it seem just a little farfetched that Wex followed the group across half of the North without being observed or sensed even once, then waltzed into WH?

No, I think it’s that Rickon is five, so he’s not going to accomplish anything himself. So the show decided they could kill him off without changing the plot that much.