Nightbrezee, I think re-reading the whole thing is a good start.
As for the wizards ... high level casters tend to be highly indyvidualistic people. Convincing that many to work together for the epic spell is an adventure in itself. From what I remember of the Sep's party stats they have templates and often use that to mitigate buff-power. Or spam dispels on enemies approaching.
My personal mitigation on additional participans is simple. For the purpouses of Epic spell you couldn't get more extra casters than Your natural primary casting ability bonus. With int 43 it is 16 extra casters. And I never let my players treat NPC's as their "walking wallets" but like "real people".