This is a three part question.
1) Is a sentinel character across the board considered an "eligible defender" for you? I reread the original rule on sentinel and the declarer of the defender still belongs to the owner of the character. This is slightly different than ranged, which can be declared by any player making attacks. However, the character across the board, I would say, is still eligible to defend you, even though it is not "your" character.
2) in this regard, can you declare two of your characters and a sentinel character across the board when you play Hold the Line?
3) and finally, when there's a limit on character triggers of "once per", unless it says "for the group", it has been ruled that if another player takes control of the character, they can trigger it for themselves for the turn/phase/game (akin to Loreagorn). Does this mean, barring that the other two answers are "yes", that Spirit Beregond could be triggered twice (once per player) if a player across the board plays "Hold the Line"?
I know that is a lot of thought going into one round of threat reduction, but figured I'd ask anyway.