So far, I have understood point C3 to mean that the reference height (street level at the building center) is only used to determine/set the maximum heights of the building.
You understood that correctly. That fixes all your reference heights, whether you apply that to your finished floor level of the ground floor or not. You can only influence exactly where this height is located (from which your ridge height, etc., is then measured) by changing the position of your building center. Whether you expose the tooth neck of your basement or raise the terrain up to the parapet height of the upper floor: you cannot "design" your building height from the street this way.
As I understand you, you would also relate that to any terrain heights. Including ones that we as developers would like to change later, right?
You misunderstood me there: this height specification does not fix your terrain profile. Through it, you simply cannot shift reference heights.
In the schematic section drawing, the "natural terrain" (grey) is shown simultaneously with the "terrain modeling" (black). But whether these are just proposals, or if the terrain will actually be modeled that way during development, I don’t know.
The schematic section drawing—shown for several roof shapes—always represents the same:
On the left, you see the mountainside lots, with the building envelopes shown in blue dashed lines, where the mountainside houses are recommended to be positioned roughly in the center. Next comes the beige bar symbolizing the planned street, whose road centerline follows the natural terrain. On the right, you then see the valley-side lots. The symbolic buildings stand roadside at the building boundary.
The actual natural terrain profile is fictitiously drawn as a straight (i.e., evenly) descending dashed line. The terrain modeling (meaning the modification by the developers) assumes in the drawing that the terrain is altered from the street edge in a straight line rising to the finished floor level of the respective entrance story, and within the building depth rises by one story height. On the mountain side, you enter on the basement level, on the valley side at the ground floor.
It is advisable to follow this proposal, otherwise, you would have a height difference across the neighbor that would need to be properly sloped or supported. With this proposal, the mountainside developers already lie significantly above the reference height line at the basement floor level. To prevent them from losing a story as a result, they are granted 4 m more ridge/gable height. This could be a point of dispute that might change the plan until it becomes legally binding.
In C4, I see no specification regarding garage height, so different terrain heights between neighboring lots would not lead to flatter garages per se—but this may already be regulated by higher-level building laws and therefore might not have to be mentioned in the development plan.
My advice—but don’t forget: I am a business consultant, not a surveyor or building law expert—would be: orient yourself to the proposal in the development plan. As a valley-side adjoining owner: plan your finished floor level of the ground floor approximately at street level. You must make the terrain modeling modifications yourself; the municipality simply follows the planned street alignment and nothing else. Coordinate with your neighbors. Laterally, you will roughly follow the street profile because the "zero height" flows along with it (and I assume that your neighbors also do not want rain running into their front doors, but don’t want to raise terrain further beyond that).