If it's a volunteer visa from an agent, you cannot leave Thailand on your current volunteer visa. You will get denied entry while attempting to come back. If your visa was done without an agent, you can leave and come back without any problem.
I don't know about that, never had an ed visa, but this is what agent are for. If it's difficult to do something while in Thailand or you don't meet a specific criteria, sometimes agent can help you. Ofc it's tea money and everyone get their cut.
You do not need 400k baht with agent, that's the whole point of using an agent. Do not use an agent if you have the 400k baht, they will charge you agent price like 35k baht which should cost a fraction of it.
Standard is similar as my home country Canada. As for price, well, health care is free in Canada but in Thailand for wisdom tooth removal between 1k and 3k baht depending on the area. So it's really inexpensive.
There is no such thing as "having to stay out for 30 days". Benjamin is 100% correct. I hate the typical "2 weeks tourists" with zero experience who spill what they once heard and claim that it's factual. These people are cancerous to such group with many knowledgeable people who knows how ridiculous Aron statement is.