Think long and hard about trying to invest in Thailand. Your money will make more in a simple certificate of deposit than you will here. Rental income can be very seasonal and liquidity is very low.
I think it was a marriage visa and there's plenty of rain to justify a short stay to "clean up affairs" after a marriage. Just need to be honest and open.
If you are going from a long term visa to a short term visa then you are probably suspect and they'll want more details. Not knowing your home country and your Thailand travel history makes it hard to offer any further advice.
Non-o based on retirement. "O" stand for other and there's a list of "other types". You want the one that is for over 50 based on retirement from the drop down selector.
Why do you want an OA rather than O? No insurance, medical certificate or criminal background check required. And you have 30 days after you arrive to get the financial piece together. After the first year, they are effectively the same. Remember the older you get, the higher your insurance costs. Once you pass 70 it can b be cost prohibitive (from a value perspective).
It's pretty straight forward, though tedious. You need 800k in Thai bank for 2 months to get your first 1yr extension. You will need to effectively leave this in the bank (you can draw down during certain period but that only complicates the discussion). 2wks before your 90day non-o is to expire, take a trip to the bank asking them for a letter and 12 months of statements (generally they request 5 business days to process). 1wk before your non-o is to expire, go to the bank to pick up your statements and letter. Deposit a small amount and get your passbook updated. Go to immigration on that day with all required documents. Keep depositing at least 65k each calendar month forever.
When it's time to do your next 1 year extension, do the same process and ask immigration if you qualify for the 65k/month method. If yes, you can draw down your bank account to near zero if you need/want to.
Lots of steps but all very easy. Can be a bit tedious, but everything in Thailand is. Thai's love paperwork, rubber stamps and staplers!