How must the mounting positions be arranged?

... moreover, usually no nonsense or any other prohibited intent is tolerated here
I also perceived the answer as genuine, something you should simply take a moment to think about

Moreover, the point of the matter is that not all cars clog up the street.

As if the "alleged architect" wouldn't plan according to the parking space regulations,...

Can we omit the third parking space or will that be checked at some point?

Ultimately, it is a very simple risk assessment. Usually, it is checked. Now, a parking space is not necessarily a paved area, and the land use can shift somewhat in practice. Of course, there remains a risk that later someone comes up with the idea to claim the area that is no longer used for cars.
The idea of not sacrificing and sealing off any more area is basically commendable.
With a bit of chutzpah and creativity, there are solutions that won't disturb anyone further as long as you don't live in a densely populated area where street parking spaces are already scarce.

... moreover, the situation for the residents of the [Einliegerwohnung] can change very quickly. Also problematic in case of a house sale. Option: to forgo an official and approved [Einliegerwohnung]

to waive an official and approved granny flat

We also did not have the apartments approved as granny flats for the boys; our house is officially a single-family home. Technically, we have cleanly separated the units. However, in terms of taxes, subsidies (which we didn't claim anyway), and depreciation, it is disadvantageous. In the end, it was simpler and all is well.