If you extend you will probably be asked to show that you still have it. You might be asked to show proof anytime you go through immigration.
Probably not at the airport, but they can if bored or just don't like you. Just like the 20k cash they can ask you to show.
They are cracking down more and more and the DTV is under more scrutiny. Pretty sure being caught no longer qualifying is going to lead to a bad day and possible ban. Is the risk worth it to you?
You can get paid from work done before you moved. But once you move any additional work would need a permit.
Proper way would be to setup a Thai company and get a work permit via that. Not sure what your original country of origin is but if US check out the treaty of amity, it gets rid of a lot of the downsides of having a Thai company as a non-thai.
And yes patching your product would count as work. Good things there are no bugs or issues with your product.
DTV wouldn't let you work in Thailand either. It lets you work remote for a company outside of the country not in country.