I see I disagree with TheMouse, sort of. The difference is I wouldn't allow someone to defend against an action that they're not the target of, because that's essentially a free extra action. But if they're using their action on their turn they could do anything TheMouse suggested. And I forget if there's an official rile for this but I'd let someone use their action to say, "I'll wait to see what the enemy does" and then use their held action to jump in and try to disrupt them. But you need to use an action to set it up, you can't just stop people from doing stuff for free.
Which is to say that everything you just said is not according to the rules as written, but if the house rule works for you and your group, then it works for you and your group.