Could the weird choke assembly issue I described in the original post have anything to do with oil consumption? It looks like the choke does fully open after the engine has been running, only that the housing assembly is wonky.
It might. There are some engines I've worked on that an interference in the air delivery resulted in excess oil consumption. Is the valve cover breather tube connected to the air box on the exterior or interior of the air filter? If the breather tube is connected to the air box on the exterior side of filter and the oil consumption was from the breather tube, the air filter would be saturated with oil.
The only significance I can make out of the exhaust heating duct and the breather tube both being off is that it's possible that someone was working on the intake or a backfire happened.
You should check on the valve cover side of the breather tube and see if there is a reed assembly or metering plug and if that is operable.
I guess we could assume that the dipstick is the stock one and that oil levels are measured accurately? I've had a few Onans and Kohlers that I remember it was impossible to keep oil levels at "full". The engines either consumed the oil through the rings or drew it in though the breather tube. None of them retreated to the "add" level unless there was a secondary leak.
Chris B posted while I typed. The breather tube reed valve is now a top suspect.
