Thank you for this information, @James!
I’ve tried recreating problems on my own testing site but the importing features all work fine for me—I cannot recreate your issues.
Next, your plugins seem up to date so there is not a version conflict issue here.
So unfortunately, this means that the next best step I could recommend would be to test for plugin/theme conflicts. You can see the troubleshooting process for that here → https://theeventscalendar.com/knowledgebase/testing-for-conflicts/
If you can do these steps, I would recommend doing a test import after EACH step in the process. When you do this, what do you find? Do things start to work anywhere along the way? This testing may sound tedious, but it fortunately does move along a lot more quickly than it might seem.
Thank you,
George