Problems with neighbor's encroachment and building authority

and we have assumed a different border course for years

That means your retaining wall only stands behind his retaining wall because your retaining wall stands on your supposed border, but in truth rather in the middle of your property?

I do have a claim against the building authority to intervene.

Not directly.

Just because the neighbor demonstrably brazenly lies to the building authority, they can’t simply come to me out of convenience? Don’t they have to try to enforce it with the neighbor first?

The other way around: they cannot take action against his structure out of convenience against him if it stands on your property. The problem "belongs" essentially to the one on whose land it stands. For structures, the landowner is responsible – that’s why you shouldn’t let anyone build structures on your land.

Wait and see what happens in court

I do have a claim against the building authority to take action.

No. Since a civil proceeding is pending, they will also refrain from intervening here before it has been concluded. For the same reason, however, you may be able to avert imminent coercive measures by the authority. As long as it is not clarified who built what when on whose property with whose consent, the authority cannot act, unless there is imminent danger. Then they can only address the owner. Otherwise, you already have a lawyer who hopefully is well versed in construction and neighbor law and whom you can ask these questions.

There really is all sorts of things. Whether one still feels comfortable in their home because of such a story, I strongly doubt.

I would first dismantle my garden shed. and then have 10 tons of gravel dumped on my parking lot. and then see if it might suddenly be his after all.

It will surely come down to the typical situation: you win in court, but that doesn’t change anything on the ground. The rule still applies: make facts or just watch? So far it was like this: your neighbor made facts and you just watched. You will only get peace in the long term if you create facts yourselves, i.e. for example in a night-and-fog action, when the neighbors are not present, build a concrete wall exactly on the boundary that is suitable both as a fence and as a retaining wall and cannot be removed so quickly. It costs some money, but then the matter is settled. From then on, no matter what happens, always call the police immediately; they have to act first in any case. The neighbor can then sue or whatever, but it doesn’t matter to you as long as the wall stands on the boundary. You don’t necessarily have to care about the building authority, as long as your property maintains the original level.