A few random questions: It doesn't work. It affects the enemy (specifically his threat), and the enemy is immune.
Does Ithilien Tracker's ability (next enemy revealed from encounter deck does not count it's strength) apply to enemies who are immune to player card effects. To me it seems his ability still works because it is played against the encounter deck more than to an individual card.
Same thing with Thalin's ability - deal 1 damage to enemies as they are revealed by the encounter deck. Does this apply to enemies who are immune to player card effects? Sadly, it does not.
Is there an action window at the beginning of the planning phase? The rules references called 2.2 a "special action window" can actions(like Quick Strike for example) be taken during that time or is just for buying allies and attachments? Technically the first player's opportunity to play allies and attachments is the first chance to play actions -- but it's unrestricted, *any* player can play actions during this time, and may do so at any time in relation to when player one plays any allies or attachments -- go wild.
Are eagles who are discarded from play and then used as an attachment for the Eagles of the Misty mountains considered to have "left play"? I think yes but technically the card is still in play albeit facedown. The card may be present, but the character has left play.