It would take a severe reversal of fortune in ratings for the show to get cancelled before s6 as this point, though there's a lot of propbably short term silly overreaction in the US over the Red Wedding (killing three "main" characters in one show pushed the limits of what a US show is "allowed" to get away with by its audience, it was unusual enough it took them off-guard, threw them out of their comfort zone, and there was a lot of "I hated it. F*** this show" first reactions, but the uproar should abate long before the show returns, or at worst when those people realize the story goes on and new figures emerge... as it did after Ned died. Foreign audiences are in a way more used to such twists - actors leave shows far more often abroad, they don't accept to be tied to roles "forever", then they do on US TV... Beside, Talisa was a false "main character" and Cat isn't really dead).
Still not a good year for viewers obsessed with "stability" on TV shows and not liking much when shows go where they didn't expect and out of their comfort zone (Network TV and its templates of storytelling has made US audiences way too rigid, IMO), the uproar in the US over the deaths of two main characters in Downton Abbey has barely abated before the one over the RW began
For the music I wouldn't be surprised versions like this (as well as a version with the song over the credits) were tested during editing.
Most directors I work with would probably have hesitated to switch to the song with lyrics in soundtrack in that instance. Using the sound the characters are supposed to be hearing is an editing technique. In very dramatic scenes, it helps giving a feeling of direct involvement to the audience, draws you in. A soundtrack would rather play on the emotions/guide the audience, but it doesn't help making you feel you're there the way using the "real" sounds does. The scene gives me the feeling it's exactly what they wished to do, bring you there with Cat. Switching midway to a soundtrack would create an impression of stepping back and reprise your role as a viewer.
I understand what you mean, though. For book readers the song has an emotional and iconic value and a soundtrack version with lyrics would have worked over the scene and add this dimension.
For most viewers who know of the song only from ep. 8's story by Cirsei, this isn't quite the same. They also don't know what's in store when that scene with Cat and Roose happens. Their cut of the sequence, with neither a singer in the room nor switching to a soundtrack of the song was probably more powerful and effective. I really liked the dissonance, the fact the musicans played badly. It added to the feeling of wrongness and unease.
But I couldn't really be sure without watching a version of the whole sequence incl. its build up with the song in it. In my experience editing mentally (which is pretty much what director while filming) is nowhere as good as doing it for real.
Personally I think the aging of the young actors is a bit a false problem (in the sense that the viewers notice the changes but take it in stride and after a few scenes forget about it).
But I agree that for Rickon and Osha there's a good chance they resurface sooner than in the books. It's TV, they can't let arcs fall by the wayside for very long... a few episodes at most. They had the same problem with Theon this year. Intertwining the rest of ASOS with AFFC and ADWD for the next 2-3 seasons will solve a lot of it, but it's still likely they dramatize what happened to those players off-screen in the books for important/well liked characters who vanish for too long like Cat, Rickon, Theon. It's too demanding for viewers to follow otherwise, given the size of the cast and number of story lines.
It will help them keep those actors in the series as well (though I'll be surprised if they aren't forced to recast some characters at some point, especially since the cast is a lot British).
Mind you, they could very well decide to play the Rickon is now MIA card in his specific case as it was quite intentional of GRRM not to return now to this story line for now (he still won't have his five year gap that was largely meant to let Arya, Bran and Rickon age before he picked up their story lines, and he lived with it in the first two's cases, but I suspect in Rickon's case he's decided to delay his return).