So - it wasn't the decompressor valve either.
I finally decided I'd post it on FB Marketplace last night and let it go for parts - but one of the buyers was a local tree service guy, Mills' in Verner; instead of buying it he said he'd happily just take a look because he's never seen one go bad, but that they can be tempermental.
On a quick teardown, he noticed that the muffler wasn't too tight and that there was a lot of staining around the muffler. In my honesty, I hadn't really consdiered this because the saw has been professionally torn down a half dozen times already and the muffler seems like it was one of the first things to have come off.
But, after getting it nice and tight, we ran it through some 20" poplar... did about 10 cuts WOT. Got hot, but didn't stall. Same again in red oak and it finally did stall, once, before being able to restart --- but kept running.
I'm off to the property today to give it a full real-work test and will report back. But I mean, maybe I have my saw back, finally???
I'll keep this thread posted.