Why did you feel the need to use a "legal firm in Thailand" for the DTV? The requirement is that the applicant does it his/herself from OUTSIDE Thailand. This is why Thai visa agents hate the DTV: it's because they can't get a cut out of it. But here you go.
Don't over think it, just reapply somewhere else and keep it moving. Mine was cancelled without reason and I gave zero Fs. Just reapplied elsewhere after a month cooling period and was approved.
Apologies if I come off as stupid, but can't you sell off some of that stocks stuff for 10k and put it in your bank account? Then you'd have the required amount for the DTV, which is approx 17k. Again, I'm not a stocks person so I don't understand how you claim to have 40k worth of something and not be able to sell off some of it to get ACTUAL real money.