Youmu has her motivations (lol UFO's motivation-based shot types) to investigate, plus she's human enough :p (human perspective/view, human role in the spellcard battle system, etc.)
You misunderstand. The characters have their motivations to investigate; it's why they're playable characters, story-wise. I mean, what will be the link connecting the story of 10D to UFO? To PCB? It's all speculation, but hey, it's fun!
Why would Youmu and Sanae's participation mean ultimate weapon of destruction golden disaster to a writer? How does this joss what they've written? (unless CtC or TPSS is involved :p)
It all really depends. Nobody quite expected UFO's multitude of PC-98 references (the return to Makai, most of all), so fanfic writers whose setting is in Makai, or a major plot point was that Makai was no longer reachable, or whatever, now had a bunch of canon information on their hands that made their interpretations suddenly invalid. It all really depends on what information is released. But if this story arc spans back to PCB or something like that, that's got the potential to screw up a
lot of writers' storylines.
As for the PMiSS article on spirits, that's interesting stuff. More speculation fuel to be added to the fire. Namely: if Youmu is kind of a sheep herder for ghosts and spirits of different varieties, it does make me dare to hope for Kasen's appearance.
Oddly, my main reason for believing that there's a chance is not because Kasen is a recent character, or because she can destroy (evil) spirits with her bandaged arm, but for the simple fact that she doesn't have a theme as of yet. There isn't much of a precedent for me to think that she might get a theme (only Renko and Maribel of the Touhou characters not in an actual game are close to being exceptions), but I just hope that the idea of her is colorful enough in ZUN's mind to have justified him giving her a theme-- and this, in turn, makes me think of that wonderful line from ZUN's comments on Koishi's theme: "I think a garden with roses strewn all about where thorns can prick you is really fitting for a battle."