I fixed the problem myself - went into ADSIEdit, and realized the entry for "owa (Exchange Back End)" was missing entirely. Not sure how that happened, since this was a working server in production. I created it from scratch, setting most of the attributes I could based on a working server (another client) and the settings that were in this server for owa (Default Web Site). If anyone is having the same problem, the location is CN=owa (Exchange Back End),CN=HTTP,CN=Protocols,CN=SERVER,CN=Servers,CN=Admin Group Name, CN=Administrative Groups,CNDOMAIN,CN=Microsoft Exchange,CN=Services,CN=Configuration,DC=Domain,DC=com or local, or whatever.
Before I did this, I tried the /prepareAD switch, but didn't do the terms acceptance, so maybe that and prepareschema would have worked Not sure. The OWA virtual directory did exist, but recreating wouldn't have worked, since the CU19 install failed at a point that rendered the Exchange Management Shell not working.
Still having some problems with EAC and http 404 errors after login, and I've recreated the directories, and reset the authentication to Windows, which is supposed to be the fix, but so far no dice. I will figure that out. The main thing is that email is flowing again!