You were very close to doing a warm leak test...I think you should run the car in the garage for 10 min and look for any leaks. Keep the good closed and then open it like you did before valve cover gasket to ensure no more puff of smoke.

If you aren't leaking your burning. Only two ways for the oil to get out right?

Don't forget the rear main seal and the o ring on the oil filter canister binds a lot.

Did you check your coolant to be sure there is no oil in it? That car has a oil/water interchange right?

Investigate what is easiest and cheapest first no matter how unlikely they are to be cause.

 

Update- pulled the head today. Thinks looks good overall, but the exhaust valves look real oily and gooked up with carbon. The tops of the pistons still look great (I cleaned them when I rebuilt the engine, but those are the original pistons). Be happy to hear what people think of the exhaust valves.

Cylinder walls are well polished, perhaps my choice not to hone and re-ring when I had the engine out is biting me.

I'm going to re-ring it now assuming I can get the pistons out once I remove the oil pan without dropping the engine. Will hone it with a ball hone, too. I'm a little worried about doing this on the car, but I intend to be diligent about cleaning up any mess.

I bought a "delivery miles only" head from the UK since it is cheaper than buying a set of valves (Isn't that weird....). So, that also gets me tight valve guides and seals.

Will post more as I continue the work.

 

Just wanted to post a follow up. I decided I wanted to lick this problem once and for all, so pulled the head off, replaced the valve seals (with OEM instead of Fel-Pro, FWIW). I also dropped the oil pan, pulled the pistons and replaced the rings. Gap was crazy big (should be around 0.3mm, I was seeing 1-1.2mm), but rings were physically OK. I honed the block in place using a ball hone. It still measured in spec (before and after honing).

Anyway, I've got 200 miles on it now and the oil level hasn't moved. I'm afraid it is oozing a tiny bit around the main front seal (I'm getting some drops on the pavement overnight), so might have to go back and check that, but that is no biggee.