I would apply for the DTV while out of Thailand after your first visa exempt entry. There's no guarantee you'll get a second one. You need to research where you want to apply yourself but you can apply via the Jakarta embassy while you are in Bali. You need to stay in the country you apply in until the process completes or risk your application being rejected. If you also want to apply for a spouse or kids you can only do that once the primary application has completed as you need to reference that visa in the application
no such thing as a 90 day OA visa, that's the 12 month visa. Don't you mean a 90 day Non-O visa? With that, a residence certificate from immigration and apply telephone registered in his own name he should be able to open a bank account in his own name. He might still need to try and number of bank branches though
You start a company and employ 4 full time thai employees then you apply for a Non-B visa and a work permit assuming what ever it is your doing is not a restricted occupation
If you are inside the for 90 consecutive days you are legally obliged to file a report. That report can be filed up to 14 days in advance or 7 days after, but that's only to file. It doesn't remove the legal requirement to file.
You first need to work out if you are a Thai tax resident in the calendar year the money is transferred. You then need to work out whether the money you transfer is considered assessable income or not. You also need to work on your maths 12 months @ 60k per month is 720k, 12 months @ 100k per month is 1,200k. Once you've done all that you need to look at the Thai tax information to work out what exemptions you can claim, the various bands your assessable income falls into and if your income comes from a country with a Dual Tax Agreement with Thailand what if any credit you can claim for tax already paid elsewhere
If you're retired and not working and under 50 your options are limited. You could purchase a Thai Privilege visa and kick back and relax, comes in 5 year multiples. You could sign up for a soft power activity and apply for a DTV, but that only gets you 180 days at a time, albeit over a 5 year period.