If you don't already have a Thai bank account you will find it difficult to open one on a tourist or visa exempt stamp so you would be better to get the 90 day Non-O before travelling
I assume you also include tax returns or other proof of income, not just the declaration itself. The majority of the embassies that have stopped doing were just accepting a declaration without any proof so were encountering problems
I understand the purpose of the 12 months statement but don't understand why its necessary if your income is verified by your embassy. I understand why they might ask to see the proof submitted to the embassy but this just seems odd
yep I understand that, but with an income certificate there is no requirement to have a Thai bank account at all. My reference to spending was really with regards the request that if the bank book doesn't show everything they want a 12 month bank statement, I can only assume that's because they want to see all the transactions