So do D&D run the plot by GRRM before doing it? Like did they actually tell Georgie that Ramsay was going to marry Sansa, and that littlefinger would look extremely stupid?
We know that D&D communicate with George continually to make sure they don't stray too far, but how much say George has in the actual translation process is unclear.
It doesn't seem too far off base to speculate Alayne will be marrying Harry; so if D&D asked George something like 'Hey, we're running out of material for Sansa, throw us a bone?' and George revealed a few plot points like Alayne marrying Harry, then it seems logical D&D might switch the structure up a bit for logistical reasons.
I don't think they just decided Alayne was going to marry Ramsay then ran it by George to see if he's okay with it, it was probably the inverse. George told them what was going to happen to Alayne and they transcribed it a bit.
Perhaps it started with ruling out introducing Harry for logistical reasons (you would have to hire new actors to fill out the Vale storyline, new sets, new locations, etc, whereas you can just lump it together with the Northern storyline/actors/locations instead). Perhaps after ruling him out they had an idea like, 'what other marriages are coming up? Oh Ramsay and fArya!' then perhaps they started with an idea like 'wouldn't it be much more powerful if fArya was actually real?' and went from there.
The major downside to all this is that it makes Littlefinger look a total moron; but this is not as impossible to remedy as you may think.
Sure it would require a massive amount of logic bending and narrative deviation for book readers, but to have it make sense to the viewerbase at large isn't too much of a stretch:
Littlefinger's niece/daughter marries Roose Bolton's son. An alliance is brokered; Littlefinger and the Vale will help Roose's war effort, whilst Roose will promise Littlefinger something else. Who knows what.
Then all that's left is Littlefinger promising Sansa vengeance, or something, and a means to enact it. It's actually not that farfetched and I predicted it months and months ago (who is laughing now, people who disagreed with me!).
Personally, I will forgive the nonsensicalness of it all should they manage to pull it off powerfully and emotionally.
Tl;dr George probably gives a lot of input, D&D transcribe it pretty loosely this time around as there is no source material beyond what George tells them. Alayne marrying Ramsay actually can be made to make sense.