Well, most barbarians can't cast spells, and even if you have Moment of Clarity you loose all benefits of rage, including the benefit of Come and Get Me.
In general, yes this can turn into a AoO war, but the barbarian wins because he gets to always go first according to his ability. If you move and provoke and someone attacks you as AoO then you get to make an AoO that resolves first, then their AoO resolves, and then you continue whatever action you were originally doing.
According to the core rulebook, you only provoke AoO's when you start the spellcasting ation, even if it takes longer than a standard action to cast. So you can still disrupt a spell by attacking someone casting summon/enlarge but you don't get the free hit unless you're there when the caster begins the spell.
And I would say the action only gets interrupted if it is specifically interruptable like spellcasting or spell-like abilities, and that's covered under the concentration section. So under most circumstances it would not stop said action unless you drop the target into unconsciousness with your AoO.
A curious scenario came up during theorycrafting that I can't seem to clarify online with the Come and Get Me rage power and intentionally provoking attacks of opportunity by moving, charging or using combat maneuvers without feats to get extra attacks above a normal full-attack. As far as I can tell from the 3.5 analogue Robilar's Gambit, that all works, but it's not the exact same ability so I'm not convinced.
When you say "Feats to get extra attacks above a normal full-attack" you mean something like Two-weapon fighting? I am not sure how this matters but I might be missing something.
And considering the general case, what would happen if you were in the middle of an action and someone you were threatening provoked. For example, you provoked them and they decided to make a trip without Improved Trip.
If you decided to take your AoO, what would happen? Would your action still happen after you make the AoO? Or would the action be interrupted because you decided to stop what you were doing to attack? What about if you were in the middle of a full-attack, would you still keep attacking even if other actions would be interrupted?
In general, I don't think AoOs are meant to be a disadvantage for the one taking them, so they should interrupt an action only when changing some necessary conditions for the action, and this is more likely to happen to the one being hit by the attack (for example, when you successfully trip a charging foe before it reaches the intended target, the charge is interrupted). The rules say that an AoO "interrupts" the flow of actions in the sense that the AoO is resolved before the provoking action, even though is performed after it by the player.
So even if you take them during your actions, the simple act of taking an AoO should not interrupt them.In the first example, let's say that the CAGM character provokes from the enemy by moving through the foe's squares by charging. if the CAGM character is attacked with a Trip maneuver by a foe without Improved Trip, the CAGM character gets an AoO that is resolved before the Trip attempt (it would get the AoO even without CAGM though). After that, the Trip attempt is resolved and the turn of the CAGM character continues normally, if possible (if he has not been tripped). I am not sure if, assuming you have Combat Reflexes, that Trip maneuver counts as one or two opportunities for the CAGM.
In the second example, I am pretty sure you just keep attacking.
What if you were casting a spell? You're meant to 'concentrate' on the spell, but there's no check for 'Making an AoO during your spell'. A very easy case of that happening is simply someone moving past you as you were casting a summon/enlarge. Or even someone provoking from your reach weapon as they charge towards you (assuming you can wield it i.e. with Still Spell). Somewhat related to that, what about using immediate actions during spell concentration?
It says that a full-round action (or longer) spell must be cast without interruptions, but given that you need only a free hand to cast spells with somatic components (not counting Still Spells) and an AoO is a free attack, I don't think it counts as an interruption as long as you use only one hand to attack. You should ask your GM for this anyway: "attacking" may not be covered by the possible causes of interruption for a spell, but one might decide to apply a concentration check for vigorous motion when taking the AoO, since you will need to rapidly move your arm and change position to take the attack.
Concerning immediate actions during spell concentration I am not sure, but I think that would count as an interruption.
When you say "Feats to get extra attacks above a normal full-attack" you mean something like Two-weapon fighting? I am not sure how this matters but I might be missing something.I meant 'combat maneuvers without feats' to get the extra attacks. Say the CAGM Barb had 3 iteratives, he could swing twice then make a trip for the third without Improved Trip, intentionally provoking an attack which results in the CAGM character getting a full-BAB AoO instead of that -10 iterative.
Of course, you'd probably want Combat Reflexes.
I see. Well that works too. The attacks will be resolved in this order: your AoO, your foe's AoO, your trip attack. You still take your foe's attack though, unless you can prevent it, so to make this trick more fun you might want to use some attack that disrupts your target's actions as your AoO: you can try to disarm it (with Improved Disarm), or you can use Pushing Assault or the Knockback rage power to push your enemy out of reach.