[thejeff]

No. If the masters cast first in the round the slaves would not act. They are casting holy spells so that didn't happen.
If the masters cast Communion Master a round before the others cast Communion Slave, then nothing changes, except the slaves join the communion slightly later. As long as they are in when the masters cast PotS & Earthpower, it should work fine. And they are, since he can see that the slaves all have 4E.

As far as I know, there is no way to free the slaves. Once you're in a communion you're in it until you or all the masters leave the field. Even when all the masters are gone, the slaves still retain the buffs.

I'm not sure what's going on here, but it looks to me more like a problem with the spell selection algorithm, not the communion directly.