I e-mailed the previous owners hoping that they'd have some answers, but my message was returned to me. They left a phone number, though, so maybe I'll get brave and call them.
I peeked a bit under the wallpaper in the living room, and it looks like there's at least 3 layers, and possibly as many as 5, under there. I also did some peeking in the office, under the window where the wallpaper is already peeling, and this is what I found:
Clicking makes it larger...
I also looked into the bedroom closet, which is one of the few areas where you can see some plaster. This is what I saw when I looked up: Do you think that the paneling is holding up the ceiling plaster? It sure looks that way to me...!
And in the hallway by the stairs, there's a little hole where you can see under the paneling. And what do we have? More paneling. Yay.
Blogger seems to make the pictures darker than they really are, so it's had to see the paneling. But that's what's in the gap up there.
I'm seeing a lot of plaster repair in our future. On the up-side, 2 of the bedrooms and the upstairs bathroom are already done. On the down-side, that leaves the office, upstairs hallway, stairs, living room, dining room, and kitchen. And all of the ceilings of those rooms, too.
Time to start working some overtime...