Let me approach this from a different perspective that might make it easier to understand. Before we even start talking about jack handle, let’s go through the basic order of operations when you deal damage:
1) Determine attack type type (not relevant to this conversation.
2) Determine base attack (the number listed on the card that is dealing the damage).
3) Determine all modifiers, you can choose the order.
4) If the number is greater than 0, reduce hp by that amount (obviously, not if they are immune).
Let’s focus on step #3. In this step you take all modifiers into account: plus/minus of the attacker, and plus/minus of the defender. You can choose the order that all of these modifiers come into effect. For example, if the target has -1 damage reduction, and the attacker has +1 damage, you can either do +1 - 1 or -1 + 1. There will be many times will it simply will not matter, but sometimes it can. The point is that all modifiers are assessed at the same time.
Using this basic order of operations that is used in every situation in SotM, we can understand how Jack Handles work. There are essentially three options:
A) Only taking some modifiers into account:
1) Determine attack type.
2) Determine base damage.
3) Determine all modifiers that only apply to the attacker.
4) Calculate this damage and apply it to all non-villain targets
5) For each target, determine their modifiers (it makes no sense to ignore these modifiers and actually deal damage based on step #4).
6) If the number is greater than 0, reduce hp (if not immune).
I really don’t see how the card can drastically affect the order of operations, there is nothing in the card itself that says you only take certain modifiers into account first, then others later. To say this would be a big stretch.
B) Determine all modifiers, then change source:
1) Determine attack type type.
2) Determine base attack (the number listed on the card that is dealing the damage).
3) Determine all modifiers, you can choose the order.
4) If the number is greater than 0, ignore this attack, and deal that same hp that would be loss to all non-villain targets, modified for the individual targets.
In theory, this might be a good interpretation, except that we have a ruling that says it isn’t. If this was the way everything was handled, then Fixer could hit himself with Bloody Knuckles to deal everyone else 5 damage, but we know that he can’t. So, this one is out too.
C) Determine only base damage:
1) Determine attack type.
2) Determine base damage.
3) Calculate base damage and apply it to all non-villain targets.
4) Determine all modifiers for each target, you can choose the order.
5) If the number is greater than 0, reduce hp by that amount for each target (obviously, not if they are immune).
Essentially, by necessity, this is the only interpretation that fits. If so, only the first target would get the bonus damage.