Finally home...
I looked at my chains, but I don't really know the significance of all I've observed - so hopefully you smart guys will chime in:
I checked my chains for stretch by laying them out and pushing all the links together as close as I could, then I held one end still and pulled the other end to measure the overall stretch. The OEM chain, which has 2000 miles on it, stretched almost exactly 1/4 inch. The 4000 mile Tsubaki 530 o-ring chain stretched more, 5/16 inch. I didn't expect the "good" chain to be have more play than the OEM, but I don't know how much there was when it was new, so it doesn't really mean anything to me.
I doubled each chain over and held them sideways to see how much they sagged - they seemed to be about the same, and neither seemed too bad. I have no idea what's "good", or what the radius of the bend might have been...
Next, I checked for stiff links: I doubled the chains and held them, hanging vertically, by their ends with the ends touching. Then I slowly lowered one end down, keeping the two parts in contact, and observed as each link descended, took a sharp bend, and remained stationary as the rest continued down. Repeat with other end.
OEM first: It's dirty, but the links basically straighten by themselves under the weight of the chain. There is a little resistance in some, but when I examine those closely, all flex freely enough.
Tsubaki: This is harder, because even when it was new, each link had a certain stiffness, due I guess to the grease and o-rings sealing them. The links still all have that stiffness, where the weight of the hanging chain slowly pulls the links basically sort of kinda straight. But there are a couple that were properly stuck, and when I work them individually by hand, I can move them but they really don't want to go. I can't imagine that it's doing the bike or the chain any good, and I'm willing to bet that these links were the ones that felt hot after my ride. Multiple WD-40 baths for those links and lots of working between dips doesn't help too much.
So the next thing I think I'll do is put the old chain back on, as a test. But first, I'd like to clean it well and lube it as best I can: what's the best approach? Some sort of soak while it's off the bike? In what, mineral spirits? kero? If I do that, is it enough to spray lube it once it's on the bike?
And, what about the o-ring chain? I sprayed some WD-40 in a cup and tried to dip a few of the end links and get them loosened up. A lot of black stuff came off, and the links look much cleaner, but they are only marginally looser. Just how floppy should the o-ring-sealed links be?