Riello M3 (1985?) oil burner won't fire or stay lit?

System will be replaced within a month or two, so I skimped on maintenance, but it was running fine until I ran the oil tank too low (down to 2"). Added 5 gal (1") diesel fuel, this did work, but it started reluctantly -- I know you're supposed to use the RESET button sparingly, but I hit it 6-8 times before it finally lit. The added fuel may have stirred up sludge at tank bottom. Added another 5 gal diesel, then it would fire briefly and cut cut out, or not fire at all.
I've hit the RESET button way too many times.
Replaced fuel filter (it was over 1/2 water and sludge). Cleaned pump screen (didn't look too dirty but it does have a 1/16" hole in the screen!!).
Bled fuel at the pump, seemed clean but still a bit foamy, tiny bubbles; the only way I know to get the pump to run for bleeding the air is to hit RESET. This is a 2-line system without a tiger loop and about a 15' run to a buried outdoor tank (I've read that a 2-line system will prime itself).
Combustion chamber is very sooty but I don't think the cad cell is sooted over -- I pulled it out, it was clean, I put a flashlight in the combustion chamber and could see its light through the cad cell's hole.
Further air bleeding -- those foamy little bubbles -- is all I can think of, and I don't know how to make the pump run other than hitting RESET.

Unwilling to spend more money unless for bacon to feed the corgi, my backup bedroom heating system. Maybe there's corgi fur clogging the jet? (: