The Star of Helia

That's a specific group of talents that allow you to do that, like Blocker or Defensive Stance. All holding your action will do is increase your initiative next round, unless you're taking this round to aim.

Another good "defensive" option for Mages would have been spells like Bestow Defense or Defensive Shield.

Right now Romana can either attack the rat and then move, wait and see if she can take a better action later, or just move away...

You can still fire at the rat this round after having moved. My bad, I did not express myself well in the last post.

Can Kanyil use her Slayer Point to reduce the damage she took by 2 points?

Yes.

So, if I were to attempt this Shots into a turmoil, is this how it would work?
I would select a target square, count all the targets in that square, and adjacent squares, add that number as a bonus to my attack CTN, if the hit is successful, the target hit is then decided randomly (I assume by a GM die roll?).

So, if I were to select the target square to be the one north of Devin, I would get +8 to my attack CTN (all 7 rats, and Devin), or do diagonal squares count as well? In which case the attack bonus becomes +10, and NetDraeth and Feridir also become potential targets?

No. If you shoot in a group of individuals, you get a bonus of +1 per creature in that group (or cluster, or whathaveyou), who are within 1 meters of the nearest other combatant (like the rulebook mention, long, narrow formations of 1 row deep like a shield wall or a phalanx would not count for this as they're not a "cluster" of enemies).

This means that if you were to shoot at the rats in front of Devin as a turmoil, you'd get a total of +13 to your check (well, +14 really, but I assume you cannot hit yourself, so) because everyone is within 1 meter of the nearest combatant. You'd then have 7 chance out of 13 to hit a rat and 6 chances out of 13 to hit an ally. Also, if thanks to the turmoil bonus you rilled damage higher than your current maximum, it would be adjusted back down to what your current maximum damage is (barring a Coup). This is because you're not really aiming at the rat, you're just aiming at the big mass of combattants before you.

Maybe a MND+AU plus certain talents which could apply... for animal, those could be Hunter or Beast Master.

I really hope the next encounter is with something a bit bigger, trying to hit these little rats is frustrating! But, at least one of our party members is sober enough to aim straight!