1. Unfortunately, I did not understand what the problem with Path.Combine.
This is a standard class and is available for all platforms except Webplayer.
http://docs.unity3d.com/ScriptReference … mbine.html
https://msdn.microsoft.com/en-us//libra … 00%29.aspx
Prehabs you have a script «Path» in the global namespace?
Do you have such a class, delete or rename it.

2. All except the toolbar and start button, wrapped in ScrollView (img1). You will always see this button.
I do not know a way to hide it.

3. Most likely, you just do not have enough memory to do so many terrains.
Reduce the number of terrains, and/or reduce the quality of terrain maps, and/or convert your mesh in several passes.

Issue 2 related to MESToTerrains editor window.
When you dock then you cannot seee the start button.

I you do not dock it the and press start on a small mesh/terrain 512x512 then the the error messag is
"Invalid AABB rkBox UnityEditor.DockArea:OnGUI()"
and the computer eventually crashed after attemtping to generate +100 terrains