I finally solved the two remaining problems I was having with downgrading from Tahoe to Sequoia today.
The first that the plug-in “1Password Safari” was crashing the moment Safari started up. It had been working fine prior to my last reinstall. I solved the problem by completely removing both 1Password and the extension. And by completely, I mean completely, drilling down into hidden folders and ripping it all out by the roots. I also logged out of my 1Password account. After I reinstalled both apps from scratch, I was prompted to log into my account, and the extension worked again.
The Ghostscript issue took a bit more detective work. I uninstalled Homebrew and its install of Ghostscript. Then I tried a different loader (MacPorts). All that did was install it in a different location that XnView couldn’t find either…
More Googling/Redditing led me to a website where Ghostscript had been packaged into a stand-alone installer. I ran the next-to-latest version (in case it was some kind of compatibility issue) and held my breath as I rebooted and launched XnView.
It worked!