Unless you have a multiple-entry visa, then it is only good for one entry unless you buy a re-entry permit before leaving. Re-entry permits are 1000 baht for a single re-entry. It does not extend your visa, just keeps it "alive" while you are out of the kingdom.
You can't get a tourist visa at the airport. You can only get a 15 day visa on arrival IF you are from a country that qualifies for VOA. You can only get a tourist visa from a Thai embassy.
Richard Richardson Sanguila Like James Miller said, you can get a couple visas or entries back to back but eventually you will be denied admission and it's not pretty. You need to find a more appropriate method if you intend to stay in Thailand long term.
They CAN ask you. They generally DON'T ask the "casual tourist" but will ask people with a history of back-to-back entries. That said, for even "casual tourists", just because they "usually don't ask" doesn't mean they can't.
You must leave before 11:59PM on the day your entry expires. The officer may waive the first day's fee (at the discretion of the officer) but you will still be marked as overstay. You will definitely start paying fees on the 2nd day.
That is correct... But you can't enter and just stay in Thailand for 6 months as the OP stated he was doing ("Living in Thailand for 6-months straight"). Stating that the maximum stay of each entry is only 60 days (extendable to 90) is not inaccurate.
The longest time you can stay on any tourist visa is 90 days. 60 days entry on the TV, then 30 days extension of stay, then you have to leave the country and re-enter. Even though a 6-months METV is "six months validity," it's still only good for 60 days stay (extendable by another 30 days) each entry.