Considering how illusion spells focus on affecting the mind, how will it function when used against other players? Will Calm remove a player's ability to attack? Will Fear cause a player to lose control of their character as they run away? Will Frenzy make all characters appear like their enemies? Or will these spells be useless in PvP, or even absent altogether?

If they are present in the game well they be dealt with like all types of CC. You can break it with a huge cost to your stamina and will then be immune to all types of CC for 7 seconds.

I'm not familiar with the term CC in MMORPG's. What does that stand for?

Crowd Control. They are usually snares/roots/stuns/mind controls/slows things of that nature. We know that if you are CC'd (Crowd Controlled) you press the LMB first then the RMB and you lose a substantial amount of your stamina. But you do become invulnerable to CC's for 7 seconds since there are no CDs in the game. So breaking out of CC is a very strategic thing to do. Because if you break it and have no more stamina to block or dodge then you probably screwed yourself.

So to answer your OP again. We might not even know if all of those would be found in this game, but if they do show up in some type of variation then we know that we can break out of them if we want too, but at a great cost. So no CC's are never useless (most of the time) you just need to know exactly when they are appropriate to use.

Crowd Control. Sometimes known as Mezzing. (from mesmerism)