Answering my own question immediately above ...

Opening the window switch cluster showed it mostly looking great. Only a single solder joint in the very corner (I'll guess the lowest corner, where any condensation would collect) of one of the boards looked discolored. So I re-did that one, with no change.

Then I bought a replacement switch cluster on eBay. BTW, when doing that, there are (I think) two different types that use the same part number - one has a little button next to the mirror left-right selector, which I believe is for parking the mirrors. So I made sure to get one without that button, matching mine. Came out of a 2001 X5 too, just like mine.

The eBay replacement performed even worse. LOL. With that one in, it had all the problems from before, PLUS the key in the driver's door lock would no longer electrically actuate anything - door / doors / windows / and sunroof, as my original switch cluster still allowed to work. So I swapped my window switch cluster back in.

So then I went to town with INPA and Foxwell. The Foxwell turned out to be really effin good in this area. Active tests on lock / double lock / unlock failing on the rear doors, being flaky on front doors, for example. Live data clearly telling me that it thought the driver's door was always open was the strongest clue that if nothing else there was a fault in the driver's door DLA.

The plastic bracket on my original DLA that holds the end of the cable housing from the inner door latch handle had cracked many months ago. Epoxy failed to repair that, but a year or so earlier, another member here had kindly mailed me his old DLA after he saw my thread on the surgical motor repair on my DLA, hoping I could use his old one some day. So I had swapped in his, and now I know that even though it appeared to work fine at that time, it has some internal electrical problem that causes all of the problems I reported.

Still of course having my original DLA on the shelf in the retired parts museum, I took the time to do a more careful mechanical repair of that bracket, using steel and epoxy. Now that is back in, and EVERYTHING works again - central locking, remotes, etc.

And weirdly, everything now works perfectly with either my original window switch assembly, or the eBay one, that had been previously thought to have a problem.

Amazing (and sad) how a single failure in that one DLA could cause so many problems throughout the car. For example, watching the weakness of the other DLAs during the active testing had almost convinced me that they would need replacment soon, but with the original driver's DLA back in place, they work flawlessly.

Good thing BMW did not design the internet.

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2001 X5 3.0i, 203k miles, AT, owned since 2014