There's nowhere in Thai Traffic Law which states 90 days. This is an internet myth. Thai Law states "temporary visitor" - so anyone on a tourism based visa
Actually you're wrong, but I'm not going to argue. I'll just politely request you show me where in Thailand Traffic Law there is a 90-day stipulation. The international treaty is definitely 12 months
Not really. Insurance companies look only at the licence, not the IDP. I was with Allianz for over twenty years as an investigator (yeah I was one of those asses who denied claims!), and no insurance claim was ever denied because of IDP issues. The biggest problem in Thailand (and Indonesia) was people riding small motorcycles on car licences (because some countries allow this in their country). These were always denied.
No point in debating it. These are the conditions of obtaining the DTV. It's that simple and it's not restrictive for most people. If you don't have savings to this value, you're not really the type of person who can bring value to Thailand
Anonymous participant The point I'm making is only one passport is valid. Yes you can show both - the EXPIRED one, and the VALID one. The scenario you suggested initially was you would show only the EXPIRED passport, passing it off as a valid passport.
To remain in Thailand different visas have different financial requirements. In order to accurately answer your question you need to specify which visa you are using