My best pal is quite a senior Diplomat (Not Thai). I was telling him about the DTV. He was amazed about 2 things 1) That the MFA could create a visa and issue it without the interior depth. 2) That embassies do not have fill sight of the system and immigration history of all applicants.
Talk to the Plants and Herbs people on Whats-App. They are very responsive and will tell you where recent applicants have been successful. As for shorter courses there are currently no issues doing those and re-entering without further booked courses.
This is a difficult one. Was it the gym documentation the root cause or your finances or other issue? If the fault on your end the gym has no reason to refund.
Yes, if you leave on the "O" and do not have a re-entry permit it is basically cancelled. If you are applying for a DTV though I would point this out in a cover letter. The "Local Hire" initially reviewing your application at the Embassy you apply to will not likely know the nuances of all Thai visa types. PS: Why change now though? Can always get DTV towards end of "O"