IDP is only meant for temporary use, not for people living in a country, but every country can decide if they want to accept foreign drivers licences for everybody who lives in the country
The validity period of IDP is internationally agreed, how long after entry it can be used in a country, every country decides for itself. And Thai laws are difficult to find. Apart from that, insurances and rental companies can have their own rules.
I agree with you it is a bit strange that when applying for a visa that let you stay over 90 days there is not a single instruction about the TM47 90 day reporting. I know there are some people telling everybody should know the law. In Hat Yai I could just walk in and tell I had to do 90 day reporting. 5 minutes later I was outside again. What I needed was TM30, copy passport entry stamp, copy passport personal details, copy visa and filmeditie in TM47 form. I got the filmeditie in TM47 form by applying online first, sometimes that seems to work even for the first time, but it also gives you filled in TM47.
it is all about timing. If you want to stay maximum of 180 days, best do first 90 day report a bit too late within the grace period, then the next one needed is after