There is both the OA and Non O visa you can apply for both from your home country, they both have slightly different requirements of course. But you cannot apply for the OA in Thailand, where you can apply for the Non O visa inside of Thailand.
you are getting confused with the OA visa, which must be applied for in your home country by showing the required funds in your bank account and also purchasing the mandatory health insurance.
The 800,000 baht must be in your account 2 months prior to your extension and 3 months after your extension is granted and for the remainder of the year you can’t go below 400,000 in your account.
That depends on the immigration office in the province you reside in, some will require a new TM 30, some will not, always best to check with your local office and what their requirements are.
You cannot switch from a OA visa to O visa inside of Thailand. If you want to switch to the O visa, you must leave Thailand first then either purchase a 90 day O visa from a Thai Consulate/ embassy or you can return to Thailand on a 60 day visa exempt stamp and start the process from inside of Thailand.
Other way around you will get a 60 day visa exempt stamp, and you will be able to get another 30 days extension at your local immigration office for 1,900 baht, giving you a total of 90 days inside of Thailand.
Health insurance is not mandatory for your visit, but I would strongly recommend purchasing travel insurance just to be on the safe side, you never know.