Good news: got it working, and everything’s operating as expected
Bad news: I don’t know how I got it working. Unfortunately, I don’t know which of my many iterations was the successful one.
However, big thanks to Marko for being so willing to help, and I’m glad 1and1 employs someone like that! Now, if only the standard tech support folks were on the same page. I don’t expect them to know everything, but I’d rather they just say “Not sure, I’ll look into it” rather than just making up answers off the top of their head.