It may depend on which version of Attack and Hide is being used. There seems to be several variants out there. The one I looked at only seemed to change one setting. I'll take another look at it. Maybe I overlooked something.
Re: Attacking groups
The behavior you described is basic to how Oblivion detection works. When attacking a target, other NPCs get a boost to detection based on their proximity to the target, not the player. Then the other factors kick in. Because the other two Goblins didn't have line of sight, they didn't have the extra bonuses needed to detect you and were like "Hey, what's the rush?".
I did a lot of experimenting in the tutorial in the main goblin cave which was ideal because of the patrolling goblin who would circle the cave. I attacked him at the far side, the shaman and other goblin would join him at attack. If I waited and attacked him when he got to my side, the others wouldn't notice. Goblins have a really low sneak, so presumably, if I set the base sneak value of the shaman goblin really high and tried the same trick, he would spot the skirmish.