Hey.
You might want to look at the SuggestProjectileVelocity Custom Arc node.
It takes in a starting point and an end point and gives your object the exact velocity needed to travel between those two points.
Using this node and based on the logic I believe you have, you can set the End Pos while you are doing your aiming (the point where your aiming arc hits)
Then the moment you want to release your grenade, you can set its location as the Start Pos.
This should solve your problem with the minimum amount of redoing logic.
Hope it helps. 
- Dan