I found as long as I add all necessary MeshBuffers (the can also be empty at this point in time, does not seem to matter) before creation of the SMesh + SceneNodes, I can fill them later (during runtime of the program) with new vertices and indices, and the program does not crash.
Therefore I found my solution

Most likely I will rewrite the terrain code from some year ago, as it is currently very awful, and I learned a lot of stuff since this point in time.
Have a nice day.