I know but you know as well as I that many people talk about VOA in error for visa exemption. I wasn't aware that Bulgarian passport qualified for VOA. 🙏
It's not a 30 day 'visa on entry '. It's a visa exemption stamp. Allows 30 days visit, can be extended once for a further 30 days. Then you have to leave. Suggest get a 'proper' visa in your home country.
Yes, if you have a multiple entry visa. Otherwise you may get a 30 day visa exemption (depending on what passport you have). Visa exemption is extendable by a further 30 days at an immigration office. 1900 baht
Hotels will register you with immigration. Some won't bother. Don't worry, not your problem. If you are staying somewhere long-term, for example, you are renting a place, landlord should do a TM30 registering you.