This is, without question, the worst designed card in the game. At GenCon a few years ago, a player showed up for the convention exclusive event with this guy as one of his heroes. His intention was to force a game loss by driving everyone’s threat up. It was the most negative play experience I’ve ever had with this otherwise fantastic game. Doomed should have not raised other players’ threats this way.

I think this says more about that player than Grima himself. But still yeah he is more of a solo player. Unlike the Doomed events which benefit all players his Doomed will usually benefit more the Grima's player (but it can still be used to discount some global effects). —
I would honestly just houserule grima or the doom archetype, so that doomed effects which benefit only one player are only resolved by the person itself. It just doesn't feel right —
A "deep knowledge benefits every player on the table, so it makes sense that every player raises his threat. But abilities like Grimas or some dommed cards from the ringmaker cycle like Herald of Anorien are just unplayable or noticeably worse for no reason. —

Planning on use it for first time in LOTR Saga in The Battle of the Pelennor Fields and/or The Tower of Cirith Ungol to get ride of the Wraith on Wings.

These scenarios are one of the few cases where I find it useful enough. But even then, you still have to engage them before completing this quest, not easy.

16
Even in that case I am not totally sure it's worth it. Since it's a battle side quest you need to devote a large amount of attacking power to it, attacking power that could have possibly been used to kill the Wraith the old fashioned way. Helm! Helm! or Ride Them Down (or even Hunting Party) might be better options to deal with the Wraith on Wings —
  1. Can Treebeard pay for, say, Derndingle Warrior purely out of his collected resource pool if for example my only Tactics hero was captured during Escape from Dol Guldur and thus Derndingle Warrior being out-of-sphere at that time?
  2. Does Treebeard enter play exhausted even if cheated in play with say, Sneak Attack or Elf-stone?
Correct on both points —
Treebeard resources break the need for a resource match when used to play for an Ent. And he "enters play" exhausted, so no matter if you play or put into play him —

Do I understand correctly that you can use the Action: part of this card to cheat this into play without paying its resource cost (and discarding two cards, ideally Elven-lights) in such quests as Escape from Dol Guldur where there's a limit of "one ally played per round for the whole group" similarly as you can cheat allies into play with Sneak Attack or with Elf-stone in the same way?

Yep spot on. You are putting the ally into play, not playing it. So you get around the play 1 ally per round limit. Note also you can use this action any action windows you like. Doesn't have to be in planning if you don't want to do it then. —
Correct, the strength of the Jeweler is that he can jump in play for "free" in any Action Window (and yeah it bypasses the limit of Escape from Dol Guldur since he is put into play) —

Terrible to Behold? More like, terrible card to put in a deck. (Got em.)

Ok, in all serious, my main problem with this card is that you still have to exhaust a defender. To me, there's no point in canceling or preventing an attack if you still have to exhaust a defender anyway. It would have been worth it to me if you could play it after you saw the shadow card so that you could cancel an attack that you know could destroy your hero, but as written you have to play it before revealing the shadow card, so for all you know you could just be wasting a card and a resource. It seems to me like a slightly better version of A Light in the Dark that is at the same time slightly worse. Did they intend for it to be used with hero Denethor, who came out earlier in the same cycle? He's the only Noble hero in the Dream-chaser cycle. But his 3 means he probably needs this card the least out of all the heroes.