If A redirects damage to B, do you apply A’s damage modifiers or B’s damage modifiers?
Corollary: Do prevention/immunity to damage count as a damage modifier the same way increases and decreases to damage are a modifier? This corollary is moot if the answer to the original question is ‘B’.
These questions arise from a discussion on BGG: http://boardgamegeek.com/thread/726553/questions-regarding-redirect-with-reduce-immune
Hmm… I see multiple possible situations here.
My first thought is if damage is prevented (say, immunity to a damage type) then there is no damage to redirect. Such could also imply that damage redirected would be what would be dealt to the original target, allowing further reduction/augmentation after redirection.
However, it has been stated that such augmentations/reductions occur as a “static effect” and come and go as they are to be applied (which means redirecting the target means such damage modifiers that don’t apply to the new target are removed).
Based on the resolution of the “off topic” issue in the Cramped Quarters + Embued Fire topic, I would say any damage count modifiers that would apply to target A but not target B will not be in effect, and any damage count modifiers that would apply to both are not doubly applied. I"m not certain about damage type modification or damage prevention, but I would like to believe that damage prevention (or enough damage reduction to reach 0 or less before redirecting damage) results in an immediate damage cancelation, meaning there is no longer any damage to redirect. I’ll let someone more official deal with the topic of damage type modification.
Having said that, I can’t remember if it had been stated if irreducible damage can be simply prevented.
What irreducible damage you say? Wouldn’t you like to know.
I’d need a specfic situation to reply to, my instinct is to say both.
For example, a drone with +2 damage (say from Omnitron’s advanced “attack” mode being on and an obsidian field in play) attacks Tachyon who chooses to redirect the damage via Synaptic Interruption.
Drone deals 2 +1+1 = 4 damage to tachyon, which tachyon redirects with Synaptic interruption, dealing 4 + 1 (obsidian)=5 damage back to the drone.
I still hold to the thought that damage bonuses, negatives and prevention happens when the damage is resolved.
Thus if 2 damage gets redirected to someone who can not take damage but before it is resolved is redirected again 2 damage is still dealt to the new target.
If an effect grants +1 damage no matter how many times the damage is redirected or type changed the +1 is only added when the last target takes the damage.
Of course this is only my opinion. 
Giving this topic a bump because I don’t believe we’ve seen any official rulings and I think we could use it.
I am not certain that I follow all of your questions, but I will make several statements that are Official and True:
In the final example, note that this is an entirely different situation from Absolute Zero being dealt fire damage, and then dealing that much cold damage.
Note also that, if A is a Villian card and B is a Hero card in the above example, the damage dealt to C is still damage from a Villain card (since A is the source of the damage)
Does that answer all of the questions, or did I miss some?
If B is immune to damage but D has a card (Smoke Bombs) that redirects damage, is the damage still redirected?
If B reduces damage and C does not reduce damage, is the damage reduced?
My guess is that if B is immune, the damage gets cancelled before it has a chance to be redirected.
If D does not have damage reduction, then there is no damage reduction, even if B has damage reduction (B is no longer the target at this point).
The situation I meant to convey is: A targets B; D has a card (Smoke Bombs) that redirects damage from B to C.
Damage immunity only affects the final target. Meaning, Haka can swing his Mere at Baron Blade all day, but if the Mobile Defense Platform is in play, it won’t do any damage. Note that when a target is immune to damage, it’s still a target, it just cannot be dealt damage.