OK, so I stumbled upon https://barlog.rusu.info/valentin/blog/?p=300 , and comment 6 explains the problem: the whole thing's borked because "something (that i don’t have the time to investigate) has changed in qt5 and the existing kwalletd code, when linked against it, is unable to correctly handle kwl files."
All bugs are shallow in open source, eh, esr? So, following the process in comment 8, I can tricked kwalletd into running the migration process again, which automatically searches ~/.kde/share/apps/kwallet and ports them into the fancy new KWallet format. OK, so I have my passwords back now. I'll still pay the ransom to get this fixed so that it works as a reasonable person would expect it to. I'm also turning off all password management in Konqueror. There really should be something in bold lettering in the kwalletmanager5 readme warning people of this. I wasted over 6 hours of my life on this problem and I really, really don't want others wasting their lives like this, too.
