A visa grants you the full stay from the day you arrive. The day you arrive is counted as day 1 and you'll receive 89 additional days after that.
Departure information isn't mandatory for ANYONE on the TDAC. It's not a mandatory field. Don't touch it and it will remain not required. Touch it and it all becomes mandatory.
The advice is don't apply for a non-OA visa. Why are you applying for that? It's only useful in very specific circumstances and is not the proper visa for most people. The non-O is the proper visa for most people and doesn't have ANY of the requirements, just proof that you are over 50 and proof of funds, that's it.
So many reports of people only getting a month METV that applied from Japan. You need to actually do research and use the search feature of this group.
You can't use that until year 2. For the first 1-year extension immigration only accepts 800k in your Thai bank account or embassy certified income. The US embassy doesn't do that anymore, so it's only the 800k. Then during the first year as long as you get an international deposit of at least 65,000 baht every single calendar month for the 12 months before you apply for next extension, you can use that instead of the bank balance. But you need to keep your balance up to meet this year's extension requirement which is 800k for 2 months before, 3 months after extension, and then never below 400,000