Property division: Shared access via a separate plot

Apart from that, I would be interested to know (I’m not familiar with this!), how the setback areas of his house would look like, which might possibly lie on the shared plot?

No problem. Could be secured by a building encumbrance.

And what would actually happen if the neighbor sells at some point and the buyer does not want to buy the access road—can the new owner be forced to accept it including taking over maintenance obligations?

No problem. Could be locked down by land register entry.
But everything you wrote in #2 I see exactly the same way. If it can somehow be avoided, no joint access should be created. Here, in my opinion, it is even completely inappropriate since the property is then surrounded by traffic areas on three sides.

You are quite right, except that we wouldn’t be allowed to use it then, there are very few advantages for them.

You wrote that the parceling has not yet taken place. Assuming the neighbor wants more land, it could be divided differently:

The surveying costs would also be somewhat lower then.

But building encumbrance and land register entry cost money again, right? So the financial advantage of "less land" is lost again.

I put myself in the neighbor on the left side's position:

1. my property also has no access problem even without this shared driveway

2. I still have enough south garden left if I make my driveway there; I even drive out onto a quieter street there

3. it is not my problem to adjust the right side of your property to your budget.

Or as Karsten often says: §1, everyone has theirs. I see the advantage of the deal overwhelmingly or completely with you.

We found a few building gaps, but we thought if someone wants to sell, they would advertise it. But your experience obviously doesn’t confirm that!

My impression here with us is that in "older new development areas" (from the 60s to 80s) there are some building gaps where the owners simply bought the plot decades ago (or perhaps even always had the piece of land as a small field and back then came to some kind of cost agreement with the municipality regarding development, etc.) and since then "it's just there" but not needed and also not sold because that would mean more effort for the now older people than just having it lie there. When then nice, young people knock and ask if they might buy it, some may perhaps seriously think about it for the first time and come to the conclusion that none of their heirs would want it anyway or maybe the adjacent street will be renewed in the foreseeable future and one doesn’t have to "put up with" that financially if the land is not used anyway, or or or and then are finally willing to sell.

My impression here with us is that in "old new development areas" (from the 60s to 80s) there are some building gaps where the owners simply bought the plot of land decades ago

Or like in this thread:

Baulast and land register entry cost money again, don’t they?

Yes, I estimate together about €500. But I think we agree that even if you could save €5-10K with joint development, the disadvantages outweigh it.

How expensive is dividing the property? On the one hand generally and then by dividing into three parcels?

That depends on the number of divisions, land value, boundary length, and area size. Roughly estimated €2-3K.

I'll put myself in the neighbor’s shoes on the left side:

If he doesn’t have to watch every euro so closely, he might be glad to get more land for his corner bungalow – of course not the area from #1. That’s why in #19 I shifted the common boundary by 3m, which approximately corresponds to the area for the driveway.

The architect said that the right property (with a slight boundary shift) can be reduced to a maximum of 500 sqm.

You’d have to ask about the reasons. According to the facts revealed here, in my opinion, the boundary could even be shifted 3m further east again. You could also make three building plots from the total area. But for assessment the map excerpt is too small. Presumably there is no development plan, so according to §34 Baugesetzbuch (Building Code) de facto building boundaries might come into play.