Thank you to everyone who took the time to reply!
All of you make very good points. I hadn't thought of emailing faculty about this, but it makes a lot of sense, and I'm glad you pointed that out to me. I can ponder and wonder all I want about how I'd fit in with their research interests and outlooks, but unless I actually talk to them, I'll never actually know for sure. And the fact that you explicitly said that the way they will talk to me will change if I get accepted -- that was something I needed to know, because I wouldn't have reflected on that otherwise. It makes sense for them to be less open to frank discussions with someone in my current position; conversely, it makes sense for them to be more open if I get accepted.
My research area is within theoretical linguistics, but it's not as "popular" as, let's say, semantics, so I already have a somewhat limited number of universities that have people working on it -- which is both good (I have fewer places to investigate in detail) and bad (same reason). I think I'll end up applying to Uni X in the end, because (1) I really don't have that many options to choose from in the first place, and (2) the faculty working on A sound very good and there is more than one POI I'm interested in (which is good, and means I'd have a backup plan if Person X doesn't work out); that should be enough to make it worth the applications fees at the very least...