I think the clarification means you don't apply the same damage boost twice. If you are at +3 damage your strike hits for four damage, then does four to another target, that second target doesn't get the +3 a second time, it can be effected by damage reduction, and for your example increased by a seperate increase that the first damage was not modified by. It can happen, as grease monkey fist/Dual Crowbars lets you whack AZ for cold or fire damage while still hitting the villain, with Cryo chamber out it most definitely is modified.
Fixer is amazing.
We do it with jack handle too, hit AZ for healing, then whack all non-hero targets. I don't actually know how that would work out officially, but I believe Jack handle goes with what you would deal them, not what they actually take (by which I mean individual target modifiers such as DR or boost to that target would not be calculated for the damage you then throw at everyone else), so I always count the cold damage that would have been dealt to AZ before it is increased by cryo chamber and turned into healing.
If I understand what you are saying, I don't think you use Jack Handle correctly. If you have Jack Handle down, Fixer can't damage AZ. Anytime he deals damage it just goes to all non-hero targets. It is exactly as though any card which tells Mr Fixer to deal damage specified 'all non-hero targets' as the recipient.
Similarly with Dual Crowbar, modifiers to the first target don't matter at all, unless they they normally apply to the second target. It is just as though the card which told him to deal damage said "and another target" along with whatever target it actually specifies.