It's not so much the police that is the problem, as you say, money talks. It is the insurance company if you have an accident, if you are over 90 days the IDP will not be valid, so your UK licence on its own will not be valid either.
I was stopped in a police check in Roi Et a while sgo, they asked for licence and insurance, probably hoping for some T money. As soon as they saw both the licence and IDP they waved me on without bothering to look at either.
I was told (but don't know if it is correct) that it is per entry. I drove for a year on my last one as I was on an out of country non O and had to border bounce every 90 days.
If you take your UK full license + IDP, cert of residence and health certificate along to DLT you wont have to sit the test. You will have to watch the video and take the reaction tests, which TBH are easy. I got both bike and car in a couple of hours.