Imprisoning mini-game and abilities

This test map is supposed to show half of the "imprisonment" mini-game of the Garithos campaign's second map. Ordinary melee units called Antonidan Soldiers may create prisons around both enemies and allies - good thing, since there are so few of them compared to the bandit attackers. A prison simply takes the captured unit's place and wears off after a while, releasing the victim. I plan to create a variety of these "parasite units", some of which use magical attacks that each drain the mana of the imprisoned unit... It's not that hard. What bothers me is the second half of the imprisonment mini-game: the fact that specific units (temporarily called prison wagons) may pick up enemy imprisoned units, which they deliver to a certain area.

I've been working on a supposedly simple script that just picks prison units from around every wagon, then kills them cleanly (disabling the trigger that releases the imprisoned unit for a short while) and transfers their variables to the wagon's slots. Whenever I have the test puppy create a prison with this trigger on the map, I get the feedback message with blank as the unit-type, so it seems that the prison itself is chosen as the "prison_wagon" unit. This is also suggested by the fact that I get the message even though the real prison wagons are a fair distance away. Could anyone help me with this script?

I also need advice on what spell art I should use... There doesn't seem to be a way to preview spell effects directly. If anyone could provide a map made specifically for previewing special effects, or a program in which I can assess them, I'd be just as grateful.

Could you just use Devour? Order the prison wagon to drive up and "eat" an imprisoned unit. It won't have an animation, so they'll just drive up and make the target disappear, but it'd get the job done.

As far as spell art, yes, you can preview it. When picking Art, make sure that you can see the preview window on WE. When you highlight an artwork, it'll repeat itself in the preview window until you select it.