Press the insert button on your keyboard to turn overwrite on and off.
About the scarabs, firstly it would really help if you gave an example of a map where you could use scarabs as weapons, or at least more details about the map your talking about.
The easiest way to is create a fully loaded scarab by your side, wait, then revmove it.
One way that might work is moving the scarab to the location centered around the mage (the person fireing the scarab), move a small location around the unit you want to fire the scarab at (just say center location "fire scarab" around men owned by foe at "larger location that centers around the guy your fireing the scarab from"), then Run AI script
Set generic command target at "fire scarab", finally Run AI script
Make these units patrol to topleft corner at "Location centered around the mage".
The way I do it is different. I say move men owned by foe from "fire scarab" (centered in the same way as before) to "location where reavers are". Wait. Then move men owned by foe back to fire scarab. Finally move the scarab to "location centered around the mage".
Hope one of these confusingly stated methods helps you out. I'm not sure if the second one will work that well. My favorite is the third, becuase with hyper triggers you don't even notice the units are gone.
ADDITION:
Pretend m.r.bob said that. I just forgot to log off... DOH
