I decided to use a different route to figure out why my NPC movement wasn't working correctly, and searched the Unreal documentation to find out what AI movement requires. I found my answer fairly quickly: It needs character movement, which can only be added by creating the character as a Character blueprint rather than just a pawn. So I created a new NPC Character blueprint and added my AI controller to it, booted up the level and it worked perfectly, although the radius of movement was a bit too small, so I extended it a little. I made a copy and changed the mesh to a female character, and tested it. It looked really funny with all those characters sprinting around, so I toned down the acceleration on their character movement, thinking I still wanted them to break into a sprint if they'd been moving long enough, but for the most part stick to a casual walk. They were still sprinting, so I halved the acceleration again. They were still sprinting, so I cut it down to 10, which ended up being too slow, and I settled on 20. I also had to change the level gate so that the NPCs wouldn't accidentally send the player to the next level by walking into them, as I began placing NPCs at random locations in the map.
I then moved on to adding some NPCs to the other level I wanted to be busy. I was able to use the same model several times by changing the colours and hairstyles.
I then began designing the next level in the sequence. While testing a part of the level I realised that some levels I had might not have the code in the player character to respawn at the start, so I just went through and added it for each level that was currently in the content folder.
Note: I have been getting to bed late recently, last night because I was working on adding features to the website. That's why the dev logs have been light on content for a few days. On the plus side, it has proven to me that I was onto something with my new lifestyle guidelines I'm planning to publish once I have enough evidence. While I was following them, my productivity and happiness were both higher than they've ever been, and now I've stopped, they've dropped back to their previous levels.