Thank you so much for your extensive testing and very helpful replies!
I didn't see them until now because I must not have email notifications set up and I put this project aside.
Since SiteJet is the problem and there's no particular reason I used it, I will use something else. I only need a static single page website so am looking at the options mentioned in another thread on lower resource alternatives to Wordpress: https://wiki.helionet.org/WordPress#WordPress_Alternatives (old wiki links don't work?) The file listing when there's no site installed is fine, I just mentioned it to describe what was happening with nothing installed there.
I think editing the index.html directly seems the simplest, especially now that I connected to the site with FTP. Since I'm older than the web, when I think of making a webpage, I think of using an editor on my computer, then uploading it to the server. That alternatives list makes a point that Publii "is local" i.e. on your personal device? Since the wiki is about a server, that's not 100% clear to me and it seems some of the other options also run on the editor's computer and not the server. But some do run on the server, but are not in Softculous. That's all a different question, but is where I am now.
Part of the reason I put this project aside is that I paid for a customization to Easy!Appointments and was waiting for that to be finished. When it was, I initially tried to copy the files to the server using the file manager GUI. That was going to take many hours. Instead I copied the zip file then unzipped on the server, over top of the Softculous installation of Easy!Appointments, reasoning that I wanted to keep the dependencies and not uninstall E!A. That may or may not have worked. I have encountered errors that sound related to the customization so I'm in contact with the author about that. Now that I'm getting closer to the real set up I should revisit this site interaction question, since I may want to reinstall and/or reset to convert to a static site.