Looks like a pretty solid composition, even more so when 3.1 hits and your Frost Mage will get Replenishment. You mention it's primarily for PvE so my question is: Why no Paladin tank? Or do you reckon Unholy Blight and DnD will be enough help with the AoE groups?

On the CoE vs. Earth and Moon matter I'd say that Earth and Moon is superior in that way that it is applied through your main attacks and also gives you a passive 3% spell damage whereas CoE is detracting from the Warlocks damage in the sense that he can't use CoA.

As for dropping the Druid for a Shadow Priest, I personnaly wouldn't do it. The only thing a Shadow Priest really brings is Fortitude and Vampiric Embrace (and Divine Spirit in 3.1). But the Warlock would then have too use CoE. So I suppose it comes down to survivability vs. damage.