Update on our mushroom situation:
The real estate agent just came out and took a look. She said she has heard of this before and seen it once (not in this house). The mushrooms are non-toxic unless eaten and the spores should be no more harmful than common dust. That's something, at least. Now the fun continues, as appointments are scheduled for them to rip out the carpet and built-in seating, professionally clean, reseal the floor from inside and out, install new flooring, etc. This means days and days (weeks?) of me needing to hang around for these appointments too. Joy. I'm trying to be positive. Throwing a lot of anger and frustration at this won't solve it any faster. On the upside, the room is officially decluttered. There's nothing in there right now except for the mushrooms.