Well, OK, but that info appearing in setup doesn't help much when the first error stopped me from even getting to the setup screen. My fault, OK, lesson learned.
As for the upload, perhaps it is time to review the upgrade instructions.
1 & 2: backups performed, fine.
3: All files (not folders) in the remote home directory deleted. All. If this is too strict an interpretation of "remove the files" - including the existing .htaccess file - that might be problem #1, and needs clarification. Also, step 3 indicates that I will need the info out of my zp-config file, which doesn't appear to actually be the case going from 1.2.6 to 1.3, as it seems to grab this from the zp-config file that is in the zp-data folder. At any rate, deleted zp-core folder per instructions.
4: Downloaded, unzipped, uploaded new zp-core folder and new home directory files via SecureFX. No errors or special notes from FTP software.
5: The .htaccess file. Where is this meant to come from? If I deleted the files in the remote home directory, the old one is now gone. There is not a new one in the set of local new files from the download. The closest I can find is an htaccess file (no dot) in the new zp-core folder, which I briefly imagined might be a source for the setup routine to place on the server in the remote home directory - but since I am supposed to be checking this before going to setup, this seems unlikely. So, was I intended to leave the old .htaccess in the home directory where it was? For if that is the case, the upgrade instructions might have mentioned this somewhere.
6: Setup. This is where it blows apart for me - resulting in the error already mentioned above.
What I will try tomorrow is once again (third time) removing every shred of new downloaded files from my computer in case one of them went bad in transit, downloading a fresh copy of the 1.3 package, and starting again from there. 1.2.6 is currently running without problems on my server, and I should be able to upgrade as described.
If there is anything I mentioned above that I should do differently, I'd love to know about it.
Thanks!