It seems like this should have cost zero, or come with a card draw like Mighty Warrior, Fearless Scout, Diligent Noble, Song of Eärendil, or Doughty Ranger. Even those examples usually end up as binder fodder, and this is significantly worse. This matches the (awful) template of Nor am I a Stranger (Rohan) and Elf-friend (Silvan and Noldor), which similarly should have had a card draw attached or cost reduced.

If you need the Gondor trait and Steward of Gondor isn't enough, maybe consider Elessar before In Service of the Steward.

62
The only remote time I can see this useful is to give the Gondor trait to an ally, perhaps for Sword of Morthond? —

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). —

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) —