I originally went up into the attic to pull up a board to determine what I had to work with for between-joist space for a vent / fan and light setup for the shower. Found more than I was looking for - some of the attic decking had to come out as there was no way to dry out and clean the space otherwise.
It appeared to me that the water vapor from the shower went up through the walls, passed through the insulation, touched the cold attic floor, condensed, and then fell back on the insulation. Some of it was sopping, and this was more than three days after the shower had last been used. I pulled all the boards with mildew on them and all the insulation, but the plaster under the insulation was dry. This could have been so, so much worse if the plaster was soaked and the joist rotted through. As it was, some bleach, drying time, and better moisture management ought to fix the problem without requiring structural changes or bringing the ceiling down (we're planning to put fresh drywall over the old plaster for the ceiling). I can replace the cut decking with pieces off the sides, it isn't even for some reason so it won't even look strange. Won't look quite good as new due to the cut line, but it is an attic, not a living room - will be good enough and a lot better than it would look distorted by water and rot.
For day 5 I redid the register box in the room to be in the wall, not half-in-the-wall-and-half-in-the-floor, we pulled the tub, most of the floor, and some additional lathwork, and discovered that one of the major sources of moisture problems we've been finding is that the shower plumbing straight-up leaks. This has probably been like this for years and was definitely the source of some of the bulging plaster. With the tiles buckled, that allowed the moist air up the walls into the ceiling. I am not sure why this problem didn't come to a head when the previous owners had the place - must have build up over time and then maybe reached a critical point recently that made things worse faster.
Ana looked up pictures of various forms of mildew and mold on the internet and determined that ours are of non-toxic varieties (good), are surface-dwelling and generally don't compromise structures (really good), and can be killed with bleach and scraped off without replacing boards (best). So while we have already junked some scungy laths, we should be able to salvage most of the big stuff and have reason to believe that after we finish evil will not be undoing our work. We're going to bleach and scrape twice, with about a week's drying time between them when we're going to pause and go to Buffalo. First round of bleach went on today, so first round of scraping is tomorrow. Will probably work on something else as well, haven't decided what yet. With only one more day before driving to Buffalo I don't think going after the plumbing or electrical stuff is ideal, we'll see how it goes.