For the initial Non O and the first year extension the 65k per month method won’t work. That will only work for the second and subsequent years. You’ll need a lump sum of 800K and I believe Jomtien require it to be shown as coming from overseas rather than a local transfer.
Getting the bank account will be the issue and almost certainly require an agents help to do so.
Also go to the website of the embassy or consulate you will use and check their requirements. Each have their own requirements and may differ from others’ experience who applied elsewhere
A very grey area here in Thailand. On paper it’s illegal but so is not wearing a helmet on a motorcycle. Whether the police or immigration office enforce a law is up to them.
That will depend on what the agent did for you. If you provided the funds and all they did was the basic paperwork then you should be able to DIY in further extensions.
If they provided the funds for your extension then you’ll be stuck using the agent train because you didn’t follow the rules of getting the extension.
No issue at all. The need to report only applies if you have been in Thailand for 90 consecutive days. If you never stay that long then there is no need to report.