TK Grasp is the only spell in the game which has its own unique casting cost at 15 AP cast to make an attack
Nah. 15 AP is the cost of attacks made with it. The casting cost is not specified, and therefore it is default.
Again, I may interpret the rules in this case in favor of game balance. You are free to interpret them in favor of unicorns.
Like I mentioned before, all the unicorn has to do is lift the target in the air, and clamp their mouth shut.
Well, see, in the rules as written, the only conditions you can technically inflict on the enemy with Advanced Combat Actions are:
Prone (is on the ground, is easier to hit, needs to spend 20 AP to get up)
Choked (takes multiple turns. Opponent may not Break Free out of Telekinetic Choke, but they may disengage by moving out of sight)
Pinned (can't move, but can take whatever actions they want, just at a -30% penalty)
Also, Science is always used in place of unarmed/melee when making attacks for TK Grasp. Since Disarm is an advanced combat maneuver, this would qualify as an attack
[s]Well using Science for magic rolls is stupid anyway[/s]. The point was that the roll for Disarm is tied to the Science/Magic/Melee/Unarmed and not Strength.
Finally, on the matter:
There are no explicitly written rules for lifting someone into the air and completely immobilizing them with your ungodly powerful telekinesis. That's just something you made up because that's how you think the game should work. Which isn't really wrong. It just means that it's going to be up for the DM to interpret the situation and make a ruling based on that. I'm just going to repeat this again:
If the DM rules that Spell_X can do the Incredible_Feats_Of_Strength, then of course it will be overpowered. Because that's pretty much the DM's duty. The game is being ran by an intelligent (at least I hope so) human being who can always use judgment in these situations.
If you think that rules for wrestling the control of an item apply to Telekinetic Grasp, more power to you! That's your prerogative.
If I, as a DM, decide that they don't apply and use the rules for Disarm instead for reasons of game balance, then that's my prerogative.