I've always been unclear if it's 3 months after the extension is issued, or if it's 3 months after the date of the extension. I always say date of the extension to be safe. But if they take it 3 months from the date the extension is issued, you might be okay.
once you apply for the 1-year extension in Thailand, you can purchase a multiple re-entry permit. But there's no multiple entry visa available. It wouldn't matter anyways since the visa is only a 90 day visa
It's an option, but it will be cheaper and easier if you get the non-O visa before you travel. Make many things simpler. For example if you travel without a visa you can be denied boarding to your flight if you don't have proof of a ticket leaving Thailand within 60 days.
If you are trying to live in Thailand, then you will need to apply for the non-O retirement visa before you travel to Thailand. Otherwise you will not be able to open a bank account, and even if you could it would not help you if you didn't already have the visa, because in order to apply for the visa in Thailand you must show that the money was TRANSFERRED from overseas, not brought in as cash. So apply for the non-O visa, travel to Thailand. Use the visa to open a bank account and then deposit your money after you exchange it. Then after the money has been in your account for 2 full months you can apply for the 1-year extension based on retirement.
There have been multiple reports of people with a METV saying they were interrogated and some even denied entry after trying to enter the 3rd time on a multiple entry tourist visa. Immigration does not feel you are a tourist if you are trying to stay for 6+ months.