That will work only till IO thinks you are not a tourist. And when MFA thinks non-o is the correct visa in this case, IO also might come to that conclusion, but we don't know the situation.
But the administrative problem is het was not the whole time being present in the country with her, causing different end dates for the current stay. I do not judge about this, but it is good to know that in case dependants not always stay the same time in the country, this can be logical consequences. When you want to prevent this you should arrange own visa, go our of the country together, or accept you might not have possibility to extend, but only to bounce. The DTV is not meant to stay the whole time in Thailand, there is a reason the total stay has a maximum of 360 days. So therefore you cannot expect immigration to make rules comfortable for people who try to live in Thailand.
The 1 or 3 years the IDP is valid itsef has nothing to do how long a country is accepting a foreign drivers license. If Thai law does not limit this, it will be possible to use forever.
IDP is not a drivers license itself, only an explanation about your national license.
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.