I have no idea how you could possibly change unit pathing but if I were to make a Scarab, my approach would be something like this:
Check out the Carrier data editor structure and notice how its Magazine works. There are actually no Behaviors involved. It's all in the Effects.
Create your Reaver unit similarly as the Carrier. Make the Scarab in tandem of the Interceptor. The only difference will be stats and the attack. You want the Scarab to self-destruct, not shoot laser beams, so imitate Baneling Behavior.
Interceptors have flags checked so they are uncommandable and they also inherit the target of its parent unit. Make your Scarabs do the same.
Yes, if they inherited the target of their parent unit, it would go straight for the target. No need for Charge. Don't know how Carriers do it. You know how all the Interceptors will focus fire the target you tell the Carrier to attack? Do this with Scarabs. ;) Investigate the data entries for Carriers and Interceptors. I haven't done it yet myself though, but this is definitely the way to go for starters. I'm sure you'll figure it out.
I think you're recalling the behaviors of the Scarabs/Reavers wrong. It actually doesn't auto-acquire a target. If you wanted to do that, then you just do what I said, but don't inherit the parent unit's target and have the Scarab acquire a target on its own. The Reaver just picks up a target when it is in range and then launches a Scarab, which seeks the target the Reaver, the parent unit acquired. If you were to give the Reaver an order to attack a specific target, it will continue launching Scarabs at it until it dies.
first off, copy the carrier's ability (hanger), effects (Launch intceptor magazine and persistent), requirement (Arm Interceptor) and weapon.
in the requirement change any instance of interceptor to scarab. the text seen in game can be edited when you click on the "less than" node.
under your hanger ability, Ability - Info + field, you'll need to change the button, make sure the requirement is the one you set up and change the unit to your scarab at the bottom of the scroll list.
under the persistent effect change the period duration to affect time between firing. the magazine ability doesn't require any changes and might be optional.
make sure the weapon links up with the persistent ability and your done with that half.
to make the scarab I suggest looking at the baneling for the explodyness. or you could start from scratch or modify the interceptor (to save yourself some editing above). have a look at Movement - Collide on your scarab.
"When it is launched, as in BW, it sort of is not given a target, but auto-searches for one, but moves around at offsets unless the target is at a certain distance close to the scarab."
Actually, it is given a target, specifically the target the reaver wants to shoot at. The only reason the scarab moves funny is because it actually has collision (as well as a timed life) unlike all the other missiles. If you were to shoot a scarab at a target in an open area, it'd just beeline straight for the target.
Best way to make a scarab is to wait a month for them to release the reaver in the editor. =O
I know it sounds kinda shitty, but there's a few things I want, and I don't want to make a major work around, and then try and remove it in a later version, since its such a pain to find everything you changed for the workaround. So I've decided to skip those parts and just tidy up the map until the rest of the stuff is released. Expecially if you want it down 'to its last detail'