Important. Until now, many parts and elements of the game had been built in solo mode, so to speak. When I launched it in networked mode, everything was either bad or strange. Some things moved; on others, the model moved while its underlying object stayed put. Something would stand still on one screen and move, though only partly, on another, and so on. This came from my lack of understanding of the engine's networking, my hope that someone else would take responsibility for that part, and my knowledge that I'd eventually have to redo things. But I couldn't stand still just because a couple of important elements were beyond me, so I kept working.
That was the situation until now. This evening, after a long bout of head-scratching, countless diagrams, checks, errors, instructions, and manuals, I think I cracked it. Not only do login to the host, selection, the lobby, and all the surrounding mechanics work: all the models in the game level now work as they should. Everyone sees everyone else; everything can be controlled and can shoot little balls; and all of it is visible and running on both the host and the clients. This is a turning point.
At the same time, I worked out how to distribute the game through Steam or Epic, though for now I'm distributing it over the local network. A little later I'll find the IP settings, and then we'll be able to test from other computers. The last unresolved issue is multiple levels or multiple sessions, though I already have ideas about how to tackle it. That's what will let hundreds or thousands of people play, rather than just 20 to 30.