using an agent to circumvent the financial requirements involves your agent bribing an immigration officer to waive the financial requirements in your case. That is clearly illegal, but currently this is not enforced. It's similar to foreigners creating a company whose sole purpose is to allow them to buy land, which is otherwise prohibited. These things are illegal, but commonly not enforced. Not something I would want to be involved in myself.
Using an agent to circumvent financial requirements can result in immigration, who can tell how you obtained your extension, refusing to deal with you if you decide not to use an agent in the future. They will tell you to continue to use your agent. Again, not a situation I want to get involved with. The only entities I allow to hold my passport overnight are, my embassy when renewing, Thai immigration when I receive my signed retirement extension the next day, and a foreign consulate when applying for a visa. I don't willing give my passport to any agent, anywhere. A friend of mine had an agent keep his passport for over 100 days. That's not for me.
I don't find it overly burdensome to follow the rules.
4) yes, been doing that for nine years in Phuket. Never have to leave Thailand, but need a re-entry permit to leave Thailand (1,000 single, 3,800 multiple re-entry) if you want to return and resume your current permission to stay.
3) in some countries health insurance may be required for a Non-O, but when you extend that in Thailand it is not required. It is required for a Non-OA, but not for a Non-O.
2) you don't have to show 800k baht in a Thai bank to get a Non-O in Thailand or Australia. It is a requirement to have at least that much in a Thai bank for at least two (2) months when you apply for your annual retirement extension of stay.
1) Can be done within Thailand, arriving on a 60 day Tourist Visa or 30 day visa exemption. Either of which can be extended for addition 30 days at Thai immigration. During this time you should try to open a Thai bank account. Not an easy proposition because bank likes customers who already have a long term visa, a reason to arrive in Thailand with a Non-O in hand.