Left Click and Right click could be switched around, solving the close range problem as SuperBower mentioned. (Left Click for a column, right click - wall). Right Click should trigger when it clicks even a non-earth block (bukkit limitation the click only works on blocks, but why would anyone even try to collapse in the air/void of no where? You shiuld be standing on a block for cried out loud)
In all honesty. I could see combining these 2 backfiring on somebody. VERY easily. Your in a cave mining with earth tunnel. and you try to mine an ore with a pickaxe, that was on your earth tunnel slot. you just collapsed a cave on you.
Another thing is, What would you name this combination, its not Earth Tunnel or Collpase anymore.
That's not the only case to address. Lavaflow activates when I teleport throught the sign, via right click; plant passive makes it possible to break a plantblock permanently; I can shear a ship without burning to crisp by my accidental fireblast; ect; This is a
lame complain which can be easily solved without a patched fix, as Loony already suggest: remove your pickaxe to another slot or just place it in off-hand.
Not earthtunnel? No, it still is. I can collapse a wall to make myself a path. You could call it "Disinigrate", but for me Earthtunnel has a good ring to it and fits its style in that element. Besides, Surge has a shield version, and that's a misleading title when we know it doesn't surge only but it can as well shield the user.