SOLVED.
Hello Everyone,

It turned out to be the spark plugs. Somehow they were not compatible enough to keep my computer happy. I figured, that was the only thing that originally changed which hadn't been revisited... So I bought some on eBay matching the dealer part number C2A1535, which are also NKG Iridium, and changed them out two days ago - and codes were immediately gone and everything is smooth as new.

Consequently, I have 8 gently-used NKG BKR5EIX-11 that I'd be happy to mail to anyone who wants them. ~4,000 miles on them. I put the little cardboard cylinder protectors from the new ones over the old ones before putting them in the C2A1535 boxes, so they should be well-protected. Just PM me if you want them and I'll ship them out.

Thanks to everyone for your thoughts and your experience going into this. I'm really happy to have my car working like new again on the big new fuel filter you recommended and these Iridium plugs.
Garrison