1) Since Mutation says "target" for the die in the used pile, this is considered targeting. Does this mean that the die in the used pile can then be redirected? If the die in used can be redirected, would spending 1 mask redirect one of the targets or both?
No and no.
The words
“with target non-Sidekick character dice in that player’s Used Pile” are intended to specify the die that is to be swapped and not open the door for targeting effects at that die. The word “target” in this phrase is superfluous.
Wonder Woman’s global can redirect Mutation, but only the Character die in the Field Zone that is being targeted, not the Character die being pulled out of the Used Plie. The player who applies Mutation still decides which Character die is pulled out of the Used Pile.
For example, your opponent has a Sidekick and Character die [A] active, and wants to apply Mutation to a Sidekick for Character die (B) in the Used Pile. Your opponent also has another Character die [C] in the Used Pile. You have an active Sidekick and two Character dice, [D] and [E], in your Used Pile. There are three possible outcomes for Mutation being redirected:
(1) You redirect Mutation with the Wonder Woman global from your opponent's Sidekick to his active Character die [A]. This die will be sent to the Used Pile and your opponent can still choose whether they want it replaced with Character die (B) or Character die [C] from their Used Pile.
(2) You redirect Mutation from your opponent's Sidekick to
your Sidekick. Your opponent now chooses whether to swap your Sidekick with Character die [D] or Character die [E] from your Used Pile.
(3) Mutation is redirected to an active Character die whose owner has no
applicable dice in the Used Pile (e.g. all the dice are Sidekick dice, Action dice, or there are no dice in the Used Pile). In this case, Mutation fizzles, but the redirect from the Wonder Woman global itself is still legal (i.e. you can still spend the 1 mask to apply the global).
2) Suppose an opponent targets my character die in the field and a non-sidekick character die in my used pile. If I spend a Mask to change the target in the field, can I change it to an opposing character die?
You can change the target from your active Character die to an opposing active Character die (see point (2) above). In this case, your opponent would choose a die from their Used Pile to swap.
Would this swap the opponent's fielded die with my die in used or would the die in used also need to be redirected? (If it's even possible to redirect the target in used)
No, the opponent would swap an active Character die with a non-Sidekick Character die from their Used Pile.
