It's not though. It's 1900 baht. I think your guy has made an error. I was at CW last Thursday and the guy there said they'd be charging 1900 baht. Although Bangkok may be different as they tend to do things "by the book".
Money remitted to Thailand can come from any source, doesn't have to be from earnings as a DM. In my case it comes from a closed super account, so is exempt taxation
You don't pay taxes in Thailand on money earned in another country. Thailand only taxes money earned inside Thailand, and untaxed money "remitted" to Thailand.
At this point the DTV is a standard extension for 1900. Already confirmed by MFA. Also confirmed no further proof of anything required at re-entry. Just show visa