Supertapping, as indicated for lack of a better means by the double tap symbol, means the creature does not untap during its controller's next untap step. If a spell or effect would cause a currently supertapped creature to untap, it only reverts to a normal tapped state (i.e. you would have to Twiddle it twice to restore it to usable).

Oho, it's a use for Allow Custom Symbols. I haven't seen very many of those come up (which is why I haven't implemented it yet).

On this particular card, I think Llanowar Elves is a very, very good card already, and it doesn't need another option to make it even better. I'd rather see this as a 1G 1/2 (Wirewood Elf). Or at least a 0/1.

Good call on the costing.