Those who doubt that Stannis burning Shireen came right from under Martin's pen do not understand Martin. Or why he's clearly not intererested in finishing the books.
See, Stannis was once, back in Book 1, perceived by someone as a man who has principles. From that exact moment, he was doomed to be revealed as a filthy hypocrite, to fail and die in the most pathetic fashion imaginable. Well, the fact that he was a white heterosexual man without disabilities, a lawfully sired son of his father, whose immediate family line involved no incest, also played a role. But the moment of being perceived as a man with actual morals sealed his doom. Martin hates characters like with burning passion! And this doesn't just mean that they are going to die. This means he's going to write them as two-faced bastards who generally practice the direct opposite of what they preach.
With Ned Stark it was at least subtle. You could plausibly explain away his selective application of principles (on which the book explicitly calls him out through Varys) and "do-whatever-just-leave-me-in-peace" attitude towards his children and other people, except Robert, through cirtumstances.
With Robb it was less subtle, but not spelled out in plain words. Yes, Robb almost certainly was a selfish shithead, who thought with his dick, and then decided to let Edmure take the fall for it, but the narrative leaves room for doubt.
But with Stannis everything is spelled in plain words. Stannis was written as both an incarnation of hypocritical selfishness and a chud virgin from the start. In his very first appearance Stannis himself exlplains quite clearly that he's a bitter loser, who thinks that the universe owes him, and who does not care for men, gods or beasts, beyond their immediate practical usefulness to him.
The fact that despite this so many readers have latched onto Stannis even before the show came out and removed some of his unsympathetic moments, must exasperate Martin very much. And here's the root of Martin's writing block. Perhaps not the only one, but certainly a crucial one. His personal urges regarding what to do with characters and plotlines and his audience's expectations are clearly incompatible. (And now he certainly knows it.)
And well, even putting aside what actually gives Martin a boner (only metaphorical, hopefully), there is the fact that Stannis must be defeated and die for the plot. Most importantly, his mere existence obstructs Jon's plotline. If Stannis, already aware of the threat from behind the Wall, manages to beat Boltons and unite the North, that basically leaves Martin with the following options:
(1) Write Jon as a completely unironic played-straight Messiah back from the dead, who can top this feat by uniting all of Westeros against the Others by sheer charisma or something. If you think Martin would or can write this, you haven't been paying attention.
(2)Render Jon's entire storyline a pointless waste of pages.
And here we come to an objective problem facing Martin, beyond the above-mentioned writer's block. There is so much setup, that satisfactory payoff is not possible anymore, even if he decides to do his best at writing what the fans want. In this particular case either Stannis or Jon must be unceremoniously thrown into a dumpster. And everything we know about Martin's writings points to Stannis as one going to the chopping block.
The same goes for Westeros mainlaind storylines. Martin is NOT going to write Dany as a straight-up savior coming to unite the land that tore itself to pieces in a five-way war between the current pretenders, and he's NOT going to just dump Lannisters, on whom he spent so many chapters, in favor of any of the total newcomers. He has a semi-logical way to resolve that mess, and we have seen it in the show - Cersei is going to miraculously regain power and team up with Euron and shit on everyone else, including False Aegon, Tyrells, Dorne, sparrows, etc. But of course people hated it, just like they hated Stannis getting swept out of the way. So now Martin is stuck between polishing a turd and trying in vain to come up with a different resolution that would awe everyone.
