Thanks for all the suggestions. The vented loop was thoroughly clogged - freeing that eased pumping a little but still having this blow-back flushing smell.
Replaced all tubing and vented loop downstream of head (so now all plumbing is new or newish) as well.
At this point, I'm pretty sure that the head, in flush mode, is pulling waste water back in and pumping it into the bowl - which smells terrible.
I really don't care to rebuild the pump, but it seems that the replacement pump assembly (37353) is ~$160, and a whole head is only $10 more than that - so looks like this 3 year old head is getting replaced!
-dan
