I finally got around to trying out the OBi WiFi adapter today, and while I had great difficulty getting it to connect to the network, once I did, it worked like a charm.
Then out of curiosity I tried connecting it back to my regular landline and… it was able to work just fine, and it saw no backfeed at all. Meaning it was probably just having trouble due to moisture from all the rain or something, and the OBi WiFi wasn’t necessary after all.
So, I tried out the HT502, and that was also able to work just fine, and was able to make an outgoing call via pulse dialing!
Incoming calls still don’t ring quite right because as far as I can tell there’s no way to configure it with a 30Hz ring, but setting it to 25Hz gets the clapper to vibrate enough that I could theoretically tune the bells to work well enough (just more quietly). That’ll have to wait until sometime tomorrow when I’m not likely to wake up neighbors with the sound, though.
Of course if I could get the HT502 to ring at 30Hz that would be even better… I did finally find someone selling what purports to be a standard AE90 ringer and I’ve asked them for more information (like a specific part number) to see if that’d work for me. I’m not holding my breath for a response but it’d be really cool if that ends up working.
Update: The seller got back to me and confirmed that it is an AE90 ringer at the standard 20Hz. So I’ve ordered it. Another $40 spent on this silly project, oh well! It’s fun.