No doubt everyone's been waiting with bated breath to learn the outcome of my shower situation from Monday! Well, I got home Monday evening after stopping at the store to buy some more liquid drain declogger, and added a good chunk of the bottle's contents. An hour later I stared at the still-unmoved water level, and suddenly wondered whether the drain might be closed, rather than clogged.
Yup. Somehow, Bryan must've hit the drain lever while getting out of the shower, or something. It was simply... closed. I pushed the lever down, and the tub promptly drained.
Thanks to those who offered to loan me a snake! Guess I won't be needing it this time.