Those who suggested getting a more patient guy with a snake were right. The next day I called out another plumber, who spent the time required to make sure that the line was clear, not just letting some water through. In the end the guy with the hand snake who charged me $300 beat the 'well respected' company (not one of the 'rooter' people, for the record) that sent out 6 guys with a hydrojetter and camera, charged me more than a month's salary, then tried to convince me to rip up my floor and start over. They promised to give me a good price. Argh.
Anyway, problem solved. Lesson learned. Next time I'll trust my instincts without thinking twice.
Thank you all who took the time to offer suggestions.
So what was clogging the line? What about the crack? And did you ever get an inspection video of the line?
general schmutz, lots of grease.
I didn't get a video of the inspection - the guys said it was a courtesy, so couldn't do it.
From what I saw, though, the crack is pretty minor. It will of course need to be replaced eventually - or maybe lined - but for now the system works and my tenant is happy. Reckon as long as I can convince her to put bacon fat in a can and food scraps in the compost, we're good for a while.