I was playing in a local tournament yesterday, and had an opponent that played mutual cause. We were jousting so I told him he would not be able to use the card as there was not more than one opponent. Only to be told that it had been FAQ'd and was actually useable. Now I have tried to find evidence of this and so far have been unable to. What is the correct play here?

Also when I play a card that is able to discard itself, can I use that before my opponent uses a card that is able to kill characters. He was playing the wolf attachments that lower strength and the one that kills. I played a ST 2 character, he used wolf to kill. Is this correct? Could I use my ability that allows to discard after he does this and remove his icons? (Martell character)

There is no errata on the "Mutual" plots, but there is nothing on them that says they are illegal in Joust events. They do not say "play only if you have more than one opponent," so they are playable. Of course, their text starts out with the condition "IF you have more than one opponent...", so with just one opponent, the requirements to use their text will never be met. So while it is legal to play them in Joust, their text boxes may as well be blank for all the good their effects will do you. But if you want a no-text plot with mediocre stats (these days), go for it.

When you play a card in Marshaling, you take an action. After that, it is your opponents turn to take an action. So when you played that character, your first chance to use it (unless the ability is a Response to the card itself entering play) is after your opponent plays - and fully resolves - an action (or passes on the opportunity to pass on doing so). So yeah, your opponent always has the chance to kill a character before it can use its(non-Response) ability.

One of the important things to note about this game, especially in contrast to some other games like Magic: The Gathering, is that there are very few ways to interrupt the abilities of your opponents. Only passive abilities or Response abilities have the "speed" of mechanic to interfere with the resolution of something an opponent has done. It's one of the ways that the game rewards going on the offensive rather than playing defensively.

KristoffStark said:

For example, if your opponent starts to initiate a challenge by saying "I'm going to declare a power challenge with Drogon," there is absolutely nothing in this game - even a passive or a Response - that is "fast enough" to kneel Drogon before the action of initiating the challenge is complete.

General rule of thumb: Once an effect is initiated, it will resolve before any other effect (passive, Response, or standard action) can be initiated. The only exceptions to this rule of thumb are Response effects that specifically use the words "save" or "cancel." There are no stacks. There are no chains. There is no "LIFO." If something is initiated, it resolves before anyone can do anything else (except for the aforementioned "save" or "cancel" Responses). This is a very "one thing at a time" game.

The corollary to this, though, is that "turns" are by action, not by sequence. It is not your "turn" to Marshal, and then mine. Rather, it is your turn to take an action in Marshaling, then mine, then yours again, etc. until neither of us have more stuff to do. The status of "active player" only changes the options you have available to you for the actions you want to take; it does not break the basic sequence of going 1-by-1 with you, then me, then you, then me, etc. That's why if you take an action to play a character from your hand, your opponent will always get one chance to take an action of their own (that might kill the character, protect his, etc.) before you get a chance to use a non-Respone effect on that character.