there's only a few immigration offices require the 800k baht to be in the bank before applying for the Non-O visa, mostly it's a requirement for the 12 months extension
Maybe yes and maybe no. It will depend on the immigration officer you will stand in front of. I wouldn't fly though, at least if you're refused at a land border you can try another one
and from time to time the immigration office changes its requirements. Originally I didn't have to file even when returning from abroad, then I did, and now it's necessary to do it whenever I travel outside the province. I now try not to provide my passport to hotels even if they ask for it.