A Question

1. Can you make or remove cliffs using JASS?

2. When using the pick every unit and do actions action, do the actions being done to the picked units loop?

1. No.

2. Not sure what you mean. When picking units in a group, the action run once per unit. For example if you have a group with 5 units, the actions will run 5 times, and (Picked unit) refer to a different unit each time.

You should be able to make or remove rocks I believe, as you you should be able to create or remove all kind of units and doodads.