Garden path, erosion, water runoff, surface water -> problem

Hello everyone,

we have purchased a terraced house from a developer. To access the garden, a small garden path was created. This consists of compacted sand with various larger pebbles.

The problem now is that this path runs directly along the neighboring property and the height difference between our path and the neighboring property is about 30-50 cm. Our path was not paved, only a bush/tree from the old stock more or less permanently supports it.
Additionally, the path behind the houses has quite a steep slope. As a result, sand is repeatedly washed away when it rains and a roughly 1-meter-long and 10 cm-wide channel immediately clogs up. In addition, water then flows unimpeded onto the neighboring property.

How can we now "force" the developer to take action here?

Are there any regulations regarding the permanent design of paths/surfaces?

Or what about surface water on neighboring properties?

And shouldn't a water channel actually be connected to the water drainage? Ours ends directly in the soil and is supposed to seep away there.

Are there any amounts of water per sqm that one can assume during rain in different seasons and based on these, as well as the soil condition (water absorption capacity, infiltration capacity, etc.), the planning must be done?

Thanks for the help!

Baltic Sea :)