right now it's possible IF you meet certain conditions. If you don't, you can't, even if you want wanted to. However, if the ammendment mentioned in this article becomes an actual law, then it would be near to impossible for a DTVer to pay taxes in Thailand, as foreing remitted money would be exempted, and, at the same time, the DTV DOESN'T allow you to earn money from within Thailand, neither it allows you to get a work permit (you'll need to change to a different visa type then)
Participante anónimo some of them do, yes. I've heard some even charging 70,000 baht. Of course you don't need to use an expensive one, but they're out there.
If you're rejected, you'll lose around USD300-400. If you use an agent, you'll use more than that and can still be rejected. The best is to prepare very well and try by yourself.
No, the law and the article explain, both, that this is applicable to any Thai "tax resident", which is a person living over 180 days in the Kingdom. There has never been differences as per citizenship for tax matters. That's a common standard in most countries of the world (the tax resident notion, I mean, which is unrelated to citizenship or visa status).