About which track is going to shoot, it is always the nearest free to do the action. I mean, if you want to grab an enemy it will use the nearest truck with no enemies grabbed, and if you want to shoot an enemy it will use the nearest truck with an enemy grabbed (already over the truck).
About knowing if you have an enemy grabbed or not, that is confusing because the enemies are almost transparent so you can't know if you have one or not

that is because I suck as an artist.