The DTV is an eVisa and you certainly did not qualify just by being a retiree. You must have applied via a workation or soft power option. Being retired has nothing to do with the DTV.
there is indeed a medical Non-O visa, single entry 90 days. You need appointment letter from hospital/medical centre and financial proof covering medical fees and living expenses.
not at all as the tax residency requirements are crystal clear. 180 days residing within the Kingdom in a calendar year means you are a Thai tax resident. If you don’t know you are tax resident then that is simply down to your lack of knowledge.
finally you say something that I can understand. Yes tax residency refers to a tax year and not a date in that year. But, again, this is basic stuff. There is no partial tax residency, you are either resident in a tax year or not. A bit like you can’t be a little bit pregnant, you are either pregnant or not. Same with tax residency.
If your Embassy verifies your income then you can use the monthly financials. Otherwise you can switch to monthly financials only from your 2nd extension of stay. For the initial Non-O and 1st extension you will need banked funds.
having a Thai tax liability and becoming a Thai tax resident are two separate issues. You can be Thai tax resident and have no tax liability and you can also have a Thai tax liability and not be Thai tax resident. Exactly what point are to trying to make?
you automatically become tax resident by residing for 180 days inside the Kingdom in a calendar year. You absolutely become tax resident at a certain moment, the moment you reside 180 days within a calendar year. This is not a difficult concept to grasp.