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After plunging and attempting to snake the drain from ground level, and apparently making things worse, I retreated to the basement to transect the 2" PVC at a point where I could snake it both ways before it met the larger unclogged main drain.
Turns out the pipe was completely plugged along a three foot section before a turn [with gawd only knows what] and somewhat plugged for a few feet beyond that. Decades of dishwasher debris have to be a contributing factor. Either that, or some sci-fi alien intruder has been taking a dump in the sink when we're not here.
I will leave the minutiae of the unclogging process to your collective imagination. Horrible, evil smelling debris with plenty of aerosolized ejecta as I sawed the pipe. Cleaned it out and remounted with removable couplers [for ease of periodic maintenance] and it's finally fully functional again.
Despite our best efforts, there was... let's call it leakage... and cleanup promises to double our fun. I will never again complain about the money a good plumber demands.