It's hard to see who shot who when you focus on another target. Anyway, nobody is supposed to attack Xander during the droid phase and the droid aggroes independently of the usual mechanism whenever it causes damage. The only one here having any benefit of the shield is the healer.
Nah, it's easy. You just should see a steady stream of small read numbers over your head. If not, target him for split second and taunt. You can position yourself between droid and him when droid is paralised.