I did already do a reinstall earlier. It doesn't seem to have helped with anything.
My impression was that, the game being designed for multiplayer, opening a singleplayer map is kind of like joining a multiplayer game on your own machine, and that 0.0.0.0 was some special IP address for that purpose. Should it be trying to connect to 127.0.0.1 instead? If so, is there a command for that?
That aside, if I were to load Ceremony from a new game, and then use the summon command to give myself all the items that I'm supposed to have (I think I can reproduce my inventory accurately, save for the scuba pack), would that allow me to continue the campaign? Or is there some other important state data stored in the save that cannot be recreated? Because it looks like this might be my only option at this point.
Huh, so that's mystifying. Unless you just transferred your save games from the old installation, in which case yes, it most likely wouldn't work on a new reinstall either. But at least it should prevent the issue from happening again on a new playthrough.
Yes and no. The game wasn't made for multiplayer (as a matter of fact, on release there were major problems with multiplayer). The engine, however, was made with both SP and MP in mind. So both internet and local games use the same code, but they use different rules. For instance, internet games always have a server and clients. But local games only have a server (with a listen server being something in between). In internet games, there is the concept of relevancy to save bandwidth, while in local games everything is always relevant. As for 0.0.0.0 being a special IP, I haven't seen it anywhere, so I don't really think it would be the case. If the game wants to launch a local file, it doesn't use any IP at all, but rather the system path. And I don't think you can connect to 127.0.0.1 in local games, either, because you would need to set it up first. If you enter an IP to connect to, you automatically go into "client mode" and just wait for the server to handle everything.
And yes, if you summon everything after opening it, it would work just as well, except for items that have charges (like SCUBA gear that you mentioned). Make sure to launch the map with the correct difficulty, though.