DTV is basically a tourist visa so I don't think that allows for a 5 year licence. Some DLT offices won't even give you a 2 year licence unless your on a non-immigrant visa
You can get an IDP in Thailand but only if you have a Thai licence. If you want to drive in Thailand on your home country licence then you need to get the IDP in your home country before you leave
OK you need to ensure that you don't create any relationships between the German company and a Thai company but as your on a DTV that should not be possible anyway. For you personally if you are Thai tax resident and bring income into Thailand during a tax year you will have to file a Thai tax return. You may or may not have to pay tax