Risk of damage to neighbor's curbstones

Good evening everyone,
I need some more opinions.

Currently, the outdoor area is being constructed at our place. The first step is to build a retaining wall around the property (sloped property).
Today, the following problem was noticed on the side of the property adjacent to the neighbor.
The neighbor has laid a path next to the house going down from the street towards the garden and bordered it on the boundary with curbstones.
When the landscaper wanted to dig the trench for the foundation of the wall with the excavator, the first curbstone loosened because the concrete base extends onto our property. Normally, he would probably solve this by chipping off the concrete on our property at this spot.
However, the curbstones are only set in the ground at the bottom and are basically held in place only by the concrete on our property.
The fact is, if we dig the trench on the boundary, all the curbstones will be gone.

The landscaper has already pointed out that resetting the curbstones would also involve significant costs, and there is also the risk that not only the curbstones but also the laid path will come loose or suffer damage.

We are now facing the decision of what to do. Another big problem is that the house is currently empty because it is for sale. Contact details for the owners are not available. So we cannot simply seek a conversation.
We have the following options:

- proceed with the wall on the boundary as planned. Following the motto: if something on their side loosens, it is due to non-professional execution and is not our concern because we remain on our property
- proceed with the wall on the boundary as planned and repair the curbstones. Problem: we will have to bear the costs and what if more damage occurs?
- move away from the boundary by a bit. According to the landscaper's assessment, the existing wall would have to have a recessed section and we would be about 25 cm away from the boundary. These 25 cm would be lost to us on the one hand. Another problem: who takes care of maintaining this 25 cm strip since we will barely be able to access it afterwards. Furthermore, I personally dislike having to remove weeds there nicely even though we have given away this space.
As mentioned, an agreement with the neighbor is not possible.

What would you do in our place? Actually, we don’t want to lose any time because we want to make as much progress as possible before winter.

I look forward to your opinions.
Maybe someone even knows the legal side?

I would want to look into the land register with legitimate interest to see who is registered there and also put a letter on the neighbor's door. At the same time, I would send a registered letter to the old address, maybe there is a forwarding request.

Best regards
Gabriele

In general, I am always in favor of communication. The question here, of course, is what should be discussed. I firmly assume that the owners do not want to invest any more money in the outdoor area of the house to be sold. So what should be addressed?

The situation is overall quite awkward. Actually, the wall should be completely built up along the side by next week.

the first curb stone came loose because the concrete base extends onto our property.

Hey, what is it doing there? - the neighbor has to take care of his boundary reinforcement on his own land, we just recently discussed this there:

well the boundary stones have always been there, haven’t they? you can’t just damage someone else’s property and then hope for leniency. especially since the house is being sold and its value is being diminished. i would [dir als nachbar was prusten]

The last two posts illustrate the dilemma very well