Okay, last question from me: Have you tried ent_create instead of ent_createterrain?

I see how that might perhaps be what you're already doing, and the example you posted was just using ent_createterrain to be an asset-free example we can all run, but just checking. ent_createterrain inherently creates a new terrain file, while ent_create would allow you to re-use terrains. Using that in combination with ent_purge might just do it.