Who has to restore the height of the garages/parking space?

Slanted, inclined, but right-angled is already "more appealing".

The garages do not only have strip foundations, but also 1m deep earth pressure protection!

Both garages are (only) designed to be earth pressure resistant at the back.

Excavation of the Hans, yes, did write after all:
> "Everyone has commissioned the earthworks company for the excavation of 'their' area, we both have the same earthworks company (otherwise it would have been 'more complicated')."

Or what is meant?

I am repeating myself for the last time, this is not your area! This is his construction pit that he shored up on your property.

Something else. Now if the question of responsibility is settled and it is clear that your neighbor has to do it (this can be a long road and cause disputes), then he still needs your permission to place the angle retaining walls on your property, have you ever thought about that? And you refuse to grant this to him (after a long dispute).

And you do not grant him this (after a long argument).

Why should he not grant it to him? Not everyone goes through the world with outstretched elbows angrily. There will also be no long argument if starts to fill in ;)

Well, you can already assume intent on the part of neighbor B if he initially orders his garage to be earth-secured only at the back, even though he should be aware that earth will come from the left again. In this respect, a refusal to accommodate the toleration of L-shaped stone feet, etc., would be understandable, even if I would not handle it that way myself.

Yes, but eventually it will be refilled after a deadline, and B has saved his money and the industry has 2 more L-stones. B will definitely be in a dilemma or is already in a dilemma. But he tries it with A anyway.

It is not about refilling A, but about removing B. Just because everything happens in parallel in terms of time, A and B must be seen separately.

I don't know if that is legally the case. If I remember correctly (öffentliches Recht 3. Semster oder so), then by jointly commissioning a demolition contractor, they automatically formed a GbR. So it is usually not to be considered separately. One contract, one project, etc. Internally, they can then work things out. But if, for example, they had also commissioned the demolition contractor to set all the stones, then this contractor could, for example, approach either of them individually to settle the entire invoice. And so on and so forth. Therefore, I would strongly recommend to the OP not to create fait accompli yet, but rather to first consult a lawyer.