I applied in Taiwan and 500k was deposited a week before application 6 months of statements submitted. That was Dec, from recent approvals I know of it has not changed.
I had an interview in Taipei, result was 5 mins after the interview. Best to stay in the country until approved as you can be asked for proof you are still in the country or called in for an interview.
Normal activity such as salary payments are fine even if the account opened via an agent. Unaccounted for/suspicious deposits with an agent opened account or opened yourself are the ones most likely to be closed.
Malaysia is infamous for being the most demanding embassy to get approved for the DTV. You can find a lot of posts to confirm this. There are much easier embassies to apply from in SE Asia.
It depends on the embassy if the 500k needs to be seasoned for 3 months., it is not a usual requirement in the US though. Embassies in the US can take up to 4 weeks to approve, each embassy differs with the length of approval.
If you extend at an immigration office, you can stay the year without leaving but many find it easier to just do a border run and get a fresh 180-day stamp on arrival. The payment of tax depends on foreign income remitted into Thailand and you would need to seek advice if you would be liable to pay tax in Thailand. TM47 needs to be completed for the 90 day report.