I proceeded to patch the ceiling. A very crappy patching job; likely because the ceiling is ¾” plaster and I patched it with ½” drywall. Before I even got it finished, water started coming through again. Since the patching was going badly anyways, we opened it up again. We checked the toilet and monitored the drains. No source could be found.
So back came the plumber. He pocked around the pipes and looked at the bathroom. Nothing was wrong, he suggested it might be from the tub leaking behind the wall. So we cut a hole behind the shower fixtures from the second bedroom wall to monitor where the water came from.
Turns out my husband had re-caulked the tub recently. This was the cause. I re-caulked it and that was the end of that water problem.
This is how our kitchen ceiling currently looks and it will stay this way until we redo the bathroom. It feels like tempting fate to patch it at this point.
And now we know: just cause you have one giant house problem does not mean another is not around the corner. Also that my husband should not caulk.