Appreciate the input.
Faint smell of gas in oil. No indications of rich mix or unburned gas. No black smoke at all. Have had carb float issue you mentioned in the past. Not that. Agree there must be elevated crankcase pressure to have oil being pushed into the combustion chamber. Haven't done balloon but pretty sure what result would be. The question for me at this stage is the route, not so much the cause.
I didn't do the head gaskets, but the guy that did has done it many times for many years. No reason to suspect an ongoing issue there. He also replaced breather reed and gasket. Also, I tried diverting the end of the breather tube outside the air-filter housing away from the carb. as you suggested. Had no effect. Plus no oil even on the end of the tube. Breather system looks solid.
Fuel pump strikes me as one remaining possibility, but again, having no fuel-flow issues. Other than the fuel pump, that leaves the rings near as I can tell. Leak-test good, but maybe the oil-wiper ring is letting oil through. New gaskets, seals and lapped valves make combustion chamber tighter than before and maybe that's why it's gotten worse (more stress on rings, more differential pressure along sides of piston).
Bottom line is if oil is getting past the pistons and rings, regardless of the root cause, I need to re-power it. Time needed and cost of having rings done on a machine with that many hours not worth it relative to cost of a new engine, especially as it's early in grass-mowing season and here I sit with several acres.
Got my money's worth and more from the engine. Have seen CH18S's for around $1,400, supposedly in stock. May be my best option.
Thanks again.