It will be interesting to find out what the DTV is for regarding festivals etc Performers have no issue getting WP's and they are here on a weekly basis. When I worked in the corporate world we asked if we needed temporary WP's for speakers are conferences etc and were told no? We thought it was the same as performing 🙂 :-)
To quote you directly "Thailand wants to see coverage in Thailand. If your insurance covers Thailand and you have a document stating that fact, then Thailand will be happy." There is NO insurance requirement for the DTV for Thailand. Back to the drawing board.
Where is Thailand asking to see insurance for the DTV Visa - it is not. I think you need to go back to the drawing board and do some research. It would seem you are misinformed.
He DTV is just a longer term tourist visa. Would your insurance cover you on a METV for example? However, insurance companies will try anything to weasel out of paying. I would ask them upfront.