By now, I no longer want to sell.
I did not even mention this option, as the surveying costs alone amount to about €2,000-3,000. I was referring to §§912, 913 of the Building Code, according to which you are entitled to an annuity. This could also be settled with a one-time payment. Since you filed an objection in time, however, you can also demand the dismantling, which I will now assume is your goal.
The building authority has to do something, right?
No. It is very difficult to obligate the authority to take action. There is something like an inactivity lawsuit, but this only serves to enforce one's own claims, e.g. the issuance of an applied-for decision. Otherwise, you have to prove a public interest, which becomes difficult in the case of a boundary between two private properties. Clearly, there is a violation of the development plan here with a precedent effect; the authority should intervene—but you cannot rely on that and it will not help you with correcting the situation.
Both endpoints are supposed to be correct (I have not yet checked)
Before you take further steps, the local position of the boundary and the position of the retaining wall must be determined beyond doubt. If the boundary points are no longer present and/or there is no agreement between neighbors about the boundary line, you can have the boundary newly marked according to §919 of the Building Code (costs are shared by the neighbors).