The “8 minutes” thing also makes it seem like they really are only using TFA outtakes, since I can’t imagine that between TFA and TLJ there was only 8 minutes of unused footage of her. Unless RJ used everything he had and put everything else on the BR deleted scenes section (assuming they’re trying to avoid using any footage we’ve seen, even if it wasn’t actually in the films).
Well I don’t think it’s that hard to believe. When TFA came out, the editors said there was around 20-30 minutes of deleted scenes, and what did we end up with on the BD, 10? Whereas for TLJ, we did get about 30 minutes of deleted material on the BD, and I can’t find any sign or mention of any scenes that were shot but didn’t make it on disc. Of course, there are always moments and lines here and there that get trimmed and never show up in anything, so it’s possible that there’s some extra Carrie bits, but it’s probably just as possible that there isn’t.
When you factor in as well that Leia specifically had a subplot that was cut from TFA (though who knows how much there is besides what we’ve seen), plus we know that there was a lot of rewriting/reshoots/restructuring with that film (there’s at least one Leia scene, from the trailer, we know they shot that didn’t make it on disc), I think it makes sense that they’re pulling from TFA primarily. But we’ll see when it comes out. I’d love a really in depth doc on how her role (however the size) was accomplished, though I fear that’s the sort of thing they’d rather not go into.