incorrect, it is valid until it expires. There is no law regarding 90 days usage. As long as you are not on a Non Immigrant visa then the IDP is valid until it or your home country license expires. I stand to be corrected if you can state the relevant law.
there is no such animal as an international driving licence. It is an international driving permit that legalises your home country driving licence under the convention. Thailand accepts and will continue to accept IDPs.
The amount you have in your Thai bank account is irrelevant. You will pay withholding tax on any interest paid on your account at the prevailing rate, currently 15%.