Thank you all for the encouragement and advice. I will try to keep today's update short - don't want to wear out your patience!
The good news: She is still alive, no longer so horrendously stinky, and the diarrhea seems to have cleared up. I offered her a mix of corn and layer pellets, and she was eager to eat. (Thank you Rebrascora for the advice re corn - in future I'll offer it only in winter when the garden is bare and frozen ... unless the new compost heap offers enough variety.)
The bad news: She still won't / can't stand or walk. Also, I've noticed her doing an odd thing, hard to describe. Now and then she'll thrust her head forward and then flop her neck and head flat down on the ground. She'll do it just a couple times, then go back to pecking. I've googled gapeworm and I don't think it's that ... I'm starting to wonder about neurological issues. But the more I read about chicken medical emergencies, the more often I see the comment, "My chicken did that too - it's weird, I don't know what it was - then it stopped and she's fine." So this may be just one of life's mysteries.
Rebrascora, I'll examine and photograph her per your suggestions next time I check her. (I've just had her out and she's resting.) Thank you!
Would it hurt to treat with Corid anyway? I'm thinking that I need to tackle the mite issue soon - the smoke from fires is really bad right now so we're avoiding being outdoors, but in a week or so I could treat them all for mites, plus Corid, plus Valbazen, and repeat every 10 days for three sessions. Would that overload their systems? These are very mellow chickens and seem pretty unflappable ... I am IN LOVE with Sex Links!