1. Damage tokens are placed on the hero, never on Citadel plate. If Citadel Plate is removed, you do not remove any damage tokens from the hero card. Removing Citadel Plate could very well kill your hero.
2. You can exhaust any allies you control to use A Very Good Tale, regardless of sphere (blue, red, green, purple). And you can put into play allies of any sphere as well, they don’t need to be Leadership allies. And I think you mean “purple” – Leadership. “Blue” is spirit. Maybe lost in translation. We have this problem a lot when I play – my wife is Japanese, and she really wants to call Leadership the “blue” sphere.
3. Once an enemy is engaged, it stays engaged with that player unless some card effect moves it. Engagement checks are only made against enemies in the staging area.
4. When ANY character leaves play (i.e. your hero or ally, your partner’s hero or ally), the hero who has Horn of Gondor gets 1 resource. “Leaves play” is a broad term, so you can get resources when a character is defeated, when Gandalf leaves play at the end of a round, when any character leaves play after a Sneak Attack, when a character leaves play as a result of Born Aloft being discarded, etc.
5. We use A Light in the Dark to avoid attacks – sort of like a more expensive Feint, with other repercussions. Useful also in Conflict at the Carrock, to remove a troll’s buff effect for a round. Also useful for moving enemies from one side of the table to the other (i.e., use it at the end of the travel phase, before the engagement phase, and the enemy is now available for another person to engage).
6. Veteran Axehand and Guard of the Citadel are just different. GotC can quest, VA… VA has stronger attack, though. Also, VA is a dwarf and can benefit from effects targeting dwarves (like Dain’s +1 effect). GotC is sure to get some love when the new expansion comes out, which is Gondor-themed.
P.S. There is a Rules subforum, so you might want to post future rules questions in there. Anyone who is likely to answer your question is reading that forum.