I've played a 4v4 game of Mage Wars (I was curious how it would go). If you're still looking for advice about this, I can give you some.
Coordinated spellbooks are definitely a problem. For professional matches, I agree that a coordinator should be designated (or at the very least, the group needs to agree on Mages and some strategies before-hand). However, that might not be ideal for more casual matches (since having to rebuild everyone's spell books before the game will just add to the already massive amount of time the game will take). My group just used a selection of normal pre-built books (with admittedly mixed results).
The real killer for team games is definitely the time commitment. My group didn't actually finish our game (people had to leave), but we did play for a solid 4.5 hours (I estimate the game would have been over in 5.5 to 6 hours total). Now, a lot of the people involved were not very experienced with Mage Wars, and we were playing with 8 people, but even playing with 4 people is still going to take a decent amount of time.
That being said, I disagree with your idea of having simultaneous turns. That sounds like a recipe for chaos and confusion, even with just 4 players (I shudder to even think of what would happen with 6-8). I think a preferable method would be for each individual player to simply use all of their actions in a row. Then a player from the other team gets to do the same. The pattern then repeats until all players have acted. However, every player can still use their quick-cast at any normally valid time. Initiative gets traded between teams like normal, and determines which team goes first. That way, the back-and-forth gameplay of standard Mage Wars is preserved in the team variant, without dragging the game out unnecessarily.
For example, Team 1 consists of Players A, B, and C. Team 2 consists of Players D, E, and F. Team 1 starts with the initiative, so Player A goes first. Then Player D goes, followed by Player B, then Player E, and so on. The next round, Team 2 has initiative, so Player D goes first, followed by Player B. On turn 3, Player B goes first, followed by Player E.
Another way to save time would be to have individual life totals instead of shared team life. If a Mage dies, that player is out, all active spells by that Mage end, and the team will inevitably lose unless they can kill an opposing Mage really quickly (due to loss of channeling and actions). That will unfortunately lead to the end of most games being lopsided, but it will also massively cut down on game duration.
Also, for balance reasons, I think it should be mandated that each team can only have one version of any single Mage. Otherwise, you'd just see teams running with all Forcemasters and focusing on cutting a single enemy Mage to shreds before he/she could do anything.