In the case I described, my need was different. I wanted to create a terrain for buildings that were partly semi-buried and also located at different elevations .

Take a second look at my examples. What you see on the left is an entrance road to an underground parking, a level below the terrain. There is one on the right too as you can see in the first image. The horizontal walls at the top are the walls from a subway tunnel, also below the terrain. Other walls (in the center) are at the ground level (0) and one level up (5m).

The one thing that makes it a little difficult is the flaw in the plugin where it always shows and generates at level 0. This means that if you want the terrain to start below 0 you will have to temporarily raise all levels to see the walls at the correct level related to the terrain. That way the terrain and walls show in the plugin as if the terrain starts below level 0, which in my case was -50cm. The alternative is to simply start your "0-level" at a higher elevation but use it as if it's at elevation 0.

The main constraint is to be able to fix the earth height at points on the buildings in such a way that if the terrain is modified, the height of these points (linked to the buildings ) remains fixed and unchanged .

I'm not sure I understand what you mean. In the terrain editor you can change elevations but the heights of walls are not changed. What you can do is put the generated terrain object(s) on a separate level with a different elevation. This is often done to create a lake or the hole for an in-ground swimming pool.
You are right that you can't change points on a generated terrain, I don't really see the need for that since you can select the room(s) and adjust what you want. To raise a part to the top of a wall you can calculate the required height and instead of 'drawing' with the mouse you can select a point and manually enter the required elevation in the bottom-left box. I probably don't understand what you mean.

The second possible improvement would be to be able to choose a grid spacing that is a multiple of 50cm. This would solve the problem of the number of points generated for large areas (> 400 m²).

I would like this too even if the resulting terrain is not as smooth because there are fewer points. For large terrains that will make a real difference. And I can always smooth the resulting object with Blender to regain smoothness but now with fewer points.

The third possibility is the one you mentioned : “ takes a heightmap image or geo-object and converts it to a room with elevation points . ” This would be particularly useful for people wanting to create a project on land for which they have (obtained or created ) the topography elsewhere. I don't think there are many such users ; personally, that 's not my situation.

Yep that's one I would really like for another project. An absolutely necessary requirement is that it needs to generate multiple rooms in a grid you can define. That would allow you to generate a terrain the size of Australia but split in a grid of rooms that allows changing just parts of the total terrain. The terrain generator won't be able to load a complete terrain of that size.