Can a ranger use Split the tree to attack only one target (targeting the enemy and an empty square)?
Why not? The other target can be the ground, or a tree stump or something. You're throwing away half the damage, so it ain't like it's too powerful.
Just make sure not to miss the ground, or your friends will never quit making fun of you.
This sounds a bit strange. The player not only sacrifices the additional attack, but has to attack an ally with it! In any case using split the tree against a single oponent is suboptimal and not disbalanced, I don't see a reason to make it even more undesirable.
However, in the end its your call.
Because it's too blatant an attempt to manipulate the power's dumping of the worse attack roll. If you want to be able to take the highest roll, you're going to have to designate two targets which are NOT bags of rats.
Sorry, I don't buy it. Wasting half of the damage potential of the power is quite balancing enough, forcing the ranger to shoot an ally is way too punitive.