Yeah, I’m pretty overdue on video tutorials now. I’ve been working on a side project for a while now that has taken a lot more time than expected, but I’ll try to get to it soon. But I do consider ATBTT to be close to feature complete at this point, so the chance of a new major refactoring is a lot lower than what it has been before. So now is a pretty good time to start a project with it, as you suggest.
Adding characters mid-game is not difficult. The construction script of BP_Unit should take care of most of it. For placing it in initiative this just means moving the reference to the unit in the initiative order array in the Turn Manager.