There are forms for this at Immigration Office. No idea the number. Some immigration office have a special desk or window for this. They do at Chiang Mai
Plenty have left and returned same day. Some at land crossings just as long it took to leave Thailand. Enter another country, leave and come back into Thailand.
Crazy money. If an agent gives advice and quality assured your application before submission I think a 10-12k THB is fair. I woukd also expect they off a full refund if rejected as long as you were fully honest with them
Do it yourself. Ask any questions here. If you really want an agent choose one that offers money back guarantee and then only for document checking before submission. Good luck
I reckon you are less than 1% - in fact even less on a DTV. In 30 years of business trips globally, for household name companies, I was asked at a border for a business card twice. Once in the UK as a British citizen (I could not look more British) and once in Japan :-).