Hi, I have two questions.
If I am defending an attack to my friend and the shadow says "the defending player must ... " who is the defending player? The one being attacked or the controller of the character defending? If for any effect the character defending leaves play I understand that the attack is undefended and someone has to damage a hero. Who would be? The one being attacked or the controller of the character that was defending?
The defending "player" is still the one being attacked. Remember you do things in turn order so there should be no conflict. A Sentinel is just an "interrupt" that another player can use on the Defending Player's turn.
In Dol Goldur you need the three objectives to go to the last stage. Is it correct not to take them when you have enough progress in the quest card, wait until the next turn and take them in the resource phase going to the next stage in that phase? In that way we can win the game in the next quest phase without having to trigger the nasty effects of the objectives at the end of the round.
Yes, you can take them in the next action window which would occur after resources. While you are avoiding the penalties though, they could be contributing threat. Not in this case, but in others.