OK just wanted to share some thoughts on these...
1) Wall of earth is a counter so can only be used against spells as they are cast; a punch is (i believe?) a mundane action so cannot be countered with wall of earth, neither can a move.
2) I don't think discard and destroy here mean separate things mechanically. I think the distinction is to clarify that you don't take damage from boomstone after using it, or could simply be a result of the copy-paste line in the second paragraph that is on all the magic stones. I can't see a reason for removing the stones from play, and if there was an intentional "destroyed" mechanic, then there would be a destroyed pile in the rulebook or more specific text on the card.
13) Portals can be "destroyed" in a sense by summoning a wall over it.
18) Perhaps also this is for LoS blocking effects (eg dust cloud) so you can fire into or through them?
This is why I love this game, the crazy combinations that come up