Before we answer these questions, we must provide the following preface.
There are two sources of confusion or misunderstanding on this subject.
First, the context of the word “effect” in card text and previous rulings. In most cases, effect = ability. However, in other cases effect = result. When clarifying this subject, the former context (effect = ability) is applicable.
Second, that the resolution of an ability must be able to be completed or produce a result to be initiated. This incorrect application of game play stems from the first source of confusion where effect = result. The correct application is if the requirements to initiate an ability set forth below are met, this will allow the player to pay for the cost as stated in an ability.
There are two rules germane to this subject:
Each game effect has two parts—a trigger and a resolution.
Source: X-Men Forever Rule Book, Effect Timing and Resolution, p25
You may not choose to initiate an effect with no valid targets or dice to affect.
Source: X-Men Forever Rule Book, Targeting, p24
Note: This rule codified into the Rule Book the FAQ referenced in the beginning of this posting.The first rule cited can be expanded to state:
Each ability has two parts: (1) a trigger, or how the ability can be initiated, and (2) a resolution, or result once the ability has been initiated. We will focus on the first part of this rule.
To clarify the first part, a player can initiate (or trigger) an ability when (1) a cost is paid and/or the required game state occurs (as specified in the ability or as dictated by the Turn Sequence), and (2) there is a legal target.
A legal target can be defined as an object (almost always a die) that meets these requirements: (1) the target must exist for game purposes, and (2) there are no conditions prohibiting the application of the ability to the target.
Note, in card text and previous rulings, the words “legal” and “valid” are synonymous.
Using these clarifications, we can expand for completeness the second rule to state:
You may not choose to pay a cost or initiate an ability if the required game state or any legal targets do not exist. The changes and clarifications to these rules will be incorporated, along with the definitions of Game State and Legal Target, into future Rule Books.
Now let’s proceed to the questions.
1) Expanding on that notion, could a player also select a character belonging to the inactive player that "must attack" even though the current controller doesn't even have the ability to declare attackers that turn?
We do not fully understand this question as written. We assume the question to mean “Can the Active player pay for a “must attack” global ability to force a Character die belonging to the Inactive player to attack this turn?”.
No, because the Character dice in the Inactive player’s Field Zone are not legal targets – the Inactive player's dice cannot be declared as attackers. Likewise, Character dice in the Active player’s Field Zone cannot be forced to block for the same reason.
If we have misunderstood this question, please clarify.
2) Even further, suppose a player spends energy to force a character to attack. Can either player choose to use that global (or equivalent) to apply the same "must attack" effect to that target again?
Yes, albeit unorthodox. Going through the requirements for determining if a legal target in this situation exists: yes, the target exists and there are no conditions prohibiting the application of the “must attack” ability a second time on the same Character die. We understand this is a tactic to trigger other abilities by spending energy.
3) Suppose a die already has both the X-Men affiliation and the Brotherhood of Evil Mutants affiliation like Mystique: Taught by Magneto. Can the Radicalization global be used to add the X-Men or Brotherhood affiliation to such a die that already has the affiliations?
Following the clarifications above, there’s nothing preventing a player from paying for this ability to give a die an affiliation it already has. However, we feel this goes counter to the spirit of the game. Therefore, the following rule will be enacted, and will be written in future Rule Books:
You cannot pay for an ability that enables a card or die that already has the following attributes to gain them: affiliation, alignment, energy type, name/title, or subtitle.
By enacting this new rule, Mystique would not be a legal target (a condition exists – this new rule – that prohibits this ability from being applied).