If Legacy Galvanizes with Absolute Zero and Fanatic on his team, does Fanatic's exorcism start dealing a total of 4 damage (2 radiant and two melee), and can Absolute Zero deal 3 damage with his base power + isothermic transducer (1 fire damage plus 1 to himself, then 2 cold damage plus 1 to something else)? Thanks!
Yes, that's exactly right!
I’m certain you’re correct about Fanatic, and less certain about Absolute Zero, so I’ll defer on that one.
Yes, each instance of damage is affected by modifiers individually.
Okay, thanks guys :)
Side question to this: are there any instances where multiple damage types (ex; radiant and melee) are considered to be only one instance of damage?
No - each hit is a separate instance of damage, even if it's in the same power (eg Fanatic's Exorcism or Jim's Compounded Bow). As such, if someone's damage is boosted, the boost applies to both plinks of damage. The only time this wouldn't be the case is if a certain damage type is boosted and the attack consists of both that type and another - say there was something boosting all infernal damage (there isn't, but just as an example) and Jim fired his Bow - the first hit is projectile so wouldn't get a boost. But then the second hit can be any type he likes, so he decides to make it infernal, so it gets the +1 and becomes two points of damage.
Or if it says "the next instance of damage". Haka of Battle only boosts the first punch of Taiaha, for instance.
So here’s a related question: Expatriette’s Shock Rounds in conjunction with a Submachine Gun. The text reads, “When that card’s power is used to damage a target, Expatriette deals each non-hero target 1 lightning damage.” Until last night, I’d taken that to mean that the extra damage occurs per power use; so essentially, this combo does 1+1 damage to every non-hero on the board. Last night, though, I realized I could, with a clean conscience, take it to key off instances of damage, so that every time a target is damaged, every valid target takes 1 lightning damage and the total damage is something more like [1 + number of non-hero targets, assuming they all have enough HP for the experience] to each valid target.
So which is it, a flat 1 to everyone, or a scaling number of +1s?
First, apologies - I accidentally hit the Split Thread button instead of the Edit button, which has no undo and there's no way to move things back. 
Shock Rounds go off once then destroy themselves. See this thread (and hopefully the system won’t break the link by adding spaces like it did last time).
Basically you get the first instance of damage from the gun (whatever that is), Shock Rounds do a point to all non-heroes, Shock Rounds destroy themselves, then the rest of the gun’s instances of damage happen (one at a time).
So here's another interesting question: if Haka of Battld only boosts one instance of the Taiaha's damage, does that mean that Expatriette's Hollow Points only boost one instance of the Assault Rifle's Damage?
Haka of Battle says "Increase the next damage dealt by Haka by 1 for each card discarded this way."
Hollow Points says "Increase the damage of that card's power by 2."
Very different wording. 
Hollow Points affects all instances of damage from the Assault Rifle's power.
Oh I see. Makes sense.
And, to clarify again on shock rounds, it isn’t the DAMAGE that triggers the ammo; it is the use of the power itself. So, it can only go off once since you can only use each power once (except for pride-prejudice-pride, but that’s not really relevant.)