Technically immigration requires you to have an onwards ticket. They just very rarely check, and when they do it's normally because they're trying to deny you entry. For example, you spend almost a year in Thailand and they feel you're not a genuine tourist and they want to deny entry, so they find some technical reason to reject you. For 99.99% of genuine tourists they have no reason to want to deny entry
Airlines are more likely to check and it seems to depend on the airline and their policy. From Australia I never get checked these days. But from America, Canada there's many reports of people being asked. So it seems to be airline dependent.
Yes you can extend it for 30 days at immigration. Technically you require an onwards ticket within the 60 days you're allowed entry. Immigration rarely check, but some airlines do (Maybe a
*****
chance). Worst case if asked step out of line and buy a cheap ticket to a neighbouring country or a cheap onward ticket from one of the many website and you'll be able to board.
Short answer is yes. But your multiple passports will be cross matched in the system, so it will make no difference. That said there is no written limit on flying in and out visa exempt into Thailand. As long as the IO doesn't think you're abusing the visa exemption scheme to live or work in Thailand then there's no issues coming in and out of Thailand multiple times
I clicked the link and am not sure what specifically you found from it. As the other comments mentioned there is no such 90day limit rule within 6 months. You also aren't entering on a tourist visa, but via the visa exemption scheme which is now 60days. There are no issues with your planned itinerary.
If you stay over 180 days in a calendar year then yes you become a tax resident based.
However, if you have a DTA then that can override that. Especially on a tourist visa, you will likely be a tax resident of your home country as well. So will need to read your DTA and work out which country you are deemed a tax resident of.
there is no written limit on flying in visa exempt, but immigration officers can and eventually will deny entry to those who abuse the system. Sometimes it's after 4-6months, whilst some make it years before encountering issues.