1a) As of now, no. But with the recent crackdown on ED visa holders for non-attendance I can definitely see that becoming a thing at some point.

1b) Yes. They aren't going to give you a 5 year visa for having a tooth pulled.

2) This visa is for Thai cultural pursuits and that is muay thai. You might be able to find a gym that teaches other things (BJJ, MMA, ETC) but I believe the basis must be MT.

3) For MT or cooking, you have to find a school that has been approved for DTV. Not sure about medical, but I imagine they will be more lenient with who is doing the treatment as long as they are a recognized doctor or facility. You should do a search in this group for medical posts.

4) I assume you mean getting the next 180 day stamp. Just do a border run. From people's shared experiences, in country extensions are a hassle and can be even harder to get sorted than when you first applied.