well, let me say what i've tried so far
Created P12 for the scarab creation since placing scarabs will ofcourse dissapear. uhm tho P12 didn't want to listen to my setallianceto(p8, enemy); so i gave up on that very quick (yes i did set p12 to computer) and just created p7 for scarab creation. I then was ofcourse able to set p8 to enemy. but
I used this trigger:
for the locations:
Always
--
Move location P1 Dark swarm firing to unit dark swarm
preserve trigger
Always
--
Move location P1 Defiler to unit defiler
Preserve trigger
then for the scarab moving:
P1 brings atleast 1 dark swarm to anywhere
--
Give all protoss scarab owned by p7 at scarab creation to P1
Move all protoss scarab owned by p1 at scarab creation to P1 defiler
Remove all dark swarm for p1 at anywhere
(Then for the order i experimented with that an bit an month ago. i didn't use junkyard dog AI script or set generic command target.)so my next was: issue order to all protoss scarab owned by p1 attack to P1 Dark swarm firing
Preserve trigger
Lemme guess i made an fairly simple mistake again. that i'm just not seeing LoL

.
btw i did not get any scarabs.. that was the big problem actually else i could have played with the AI scripting etc an bit and found that out. i just didn't get any scarabs. 1 option is ofcourse that i can't use the give scarabs to P1? or isn't that it?
also for the record. i did use hyper triggers in this.