Terrain: Use as scenario (individual) and prefab (unchanged, reusable)

The question got two parts, but since I am not that experienced it might require only a single answer. 1. I'd like to use a basic terrain as a prefab, the prefab itself should never change regardless of events during runtime. 2. I'd like the terrain to be saved so each time a player returns to a saved scenario the player is greeted by the same environment he left.

I understand that I need to create a prefab of the terrain in question first. Consider it done. However there seems to be a problem already. When I call terr = (Terrain)Object.Instantiate(Resources.Load(pathTerrain+prefabName)); Unity throws the exception: InvalidCastException: Cannot cast from source type to destination type.

The variable terr is a Terrain object and the variable show the right values.

Moving on, regardless of the error the correct terrain is cloned (hierarchy of the scene says as much). However the prefab is also changed, which cannot be right. I know because I am only using 6 textures for the Terrain Toolkit. Whenever I register more than 6 textures in the paint tool, the Terrain Toolkit stops creating textures. Hence I need to add more textures at runtime for smalles areas like roads or rivers.

My question is, how do I instantiate a terrain prefab and make the clone independend? Follow-up: How do I store that terrain, if someone wants to save the current scenario and come back later?

I hope you can help me in this regard.

Best regards.