I would not file any complaints ! You can be denied at your first application but still approved at your second application. Maybe have someone experienced review your package, find the gaps, correct and resubmit.
Sounds good. I think that they they will slowly require this everywhere so I recommend you go to the embassy mentioned by the other posters as soon as you can. The Thai government is trying to avoid people living with no income or working illlegally in the territory hence the creation of the DTV visa and the request for proof of income in more and more embassy.
Passive income works. Proof of income usually can be pay slip, statements, outlined bank transactions coming into your account. You should write a brief letter explaining the source of your income and how you are presenting the proof.