[thejeff]

1) Slave self-buffs do not affect the communion.
Buffs remain if the caster retreats, I assume they remain if he dies.

2) Bless is not single target, so does not affect slaves. On the other hand, with 4 slaves even a H1 magepriest can divine bless.

3) I wouldn't count on the ai casting communion master/slave in any useful fashion.