I used e-visa for a land crossing. The e-visa website is very clear about which borders you can use it for. If you try to go to a border not on the list then you'll be paying again and waiting in line. The website warns you 3 times during the application that you can only use it at certain borders. Only an idiot would miss it.
You have almost zero change of getting a DTV for studying Thai language. Because Thai language was not on the official infographic that the MFA released, there have only been a couple of reports of embassies accepting it. Most embassies will either just reject your application outright, or issue a 90-day education visa instead and keep the difference in fees.
there's no health insurance issue. Just apply for the 90 day non-O visa which is what most people want anyways. It's only the OA visa that requires insurance.
And since he's talking about marriage visa and not retirement, the OA isn't even on the table.
What's required is whatever the embassy you apply at says is required. Each embassy sets their own requirement, therefore each embassy requires different things. Check with the embassy you will apply at in order to find out what they require. There have been reports of some people getting the DTV with as little as a single dentist appointment, and some people requiring a full course of treatment for some ailment. There is no answer to this question, as you'll have to check with the embassy.
he's getting married and getting a visa for that. Spending the money for a class and going through the trouble of getting an education visa makes zero sense.
Generally you'll need to use an agent to help you get the account open. Right now it's a little up in the air since that's mostly done in Pattaya but some of the agents there are no longer able to open accounts. It's possible by the time you're ready, it will be business as normal again.
Worst case scenario, you go to Laos or Cambodia and apply for the non-O visa there, then return and open your bank account for the extension.