Hiya Loco. Welcome.
1. I read in another post (initiated by Ruvion) that, in a two player game, when "Condemned by the Realm" is revealed as your plot your opponent can then kill one of your characters since he doesn't control them. However, if I don't have any characters in play, does he have to kill a character from his hand (since he doesn't yet 'control' these either)? - My assumption is NO since cards in a player's hand are not "in play" and therefore cannot be affected by 'kill'. However, I have not found a definition for 'killing' and am not certain that it only applies to characters in play (it certainly is the case during Military Challenges that you can only kill characters in play as the ruling for that challenge clearly states; but the plot card only talks about killing a character that the opponent does not control and I guess he doesn't yet control any character's in his hand?).
A general rule for this game is that cards are only actionable (meaning you can only use their effects or use another effects on them) while they are in play - unless a card's effect specifically says otherwise. So Condemned by the Realm doesn't need to specify that the chosen player can only choose to kill characters in play (that they he/she doesn't control) because that is the game's default. Generally speaking, in Game of Thrones, if a card doesn't permit something, you can't do it. This is different from some other games which take the track "if a card doesn't forbid something, you can do it."
Specifically on "Condemned by the Realm," if there are no characters in play that the chosen player does not control, there is no legal target for them to choose and nothing will actually die.
So for the most part, the definition of "kill" is card driven, not rules driven. If a card says to "kill" something, you remove it from play and (unless the card says differently, some do) put it in the Dead Pile. That's about as formal a definition as you will ever see for "kill" as a mechanic.
2. If during the Marshalling Phase I play a card, can I then kneel it and trigger its effect straight away (if it's effect is triggered during the Marshalling phase), before marshalling my next card? Example: I have two gold in my pool, so during my Marshalling phase I spend the 2 gold to bring Great Pyramid (Core Set T133) into to play, I then kneel it for its effect and then bring a character of STR 2 or less into play without having to pay any gold.
Yup. Two concepts here in general terms. First, there is no "summoning sickness" in this game. You can use the abilities on the cards you Marshall at the earliest opportunity to do so. Not only can you do things like kneel the Great Pyramid you just played, but there are many cards that can use their "after a card is played" Response to the action of actually being played themselves! The second general concept is that in the Marshaling phase, players take turns taking actions - with the understanding that a) only the Active player can spend gold to play cards from their hands and b) spending gold to play cards from your hand counts as a Marshaling action.
In practical terms, what that later bit means is that the example you describe about playing the Great Pyramid actually looks like this: 1) Spend your last 2 gold to play Great Pyramid; 2) each opponent gets the opportunity to play an action; 3) Kneel Great Pyramid to lower the cost of the next Targ character you play by 2; 4) each opponent gets the opportunity to play an action; 5) Play a Targ character with COST (you said STR; very different even though I'm sure it's a typo) 2 or less from your hand; 6) each opponent gets the opportunity to play an action
3. Finally (for now..), I just wanted to check that keywords are not abilities, therefore if a card is immune to other cards' abilities, it may still be affected by other card's keywords. Example: My opponent initiates a Military Challenge with a Sleath character and I have Quaithe of the Shadow (Core Set T115) with an attachment on it. Quaithe of the Shadow's ability does not prevent my opponent from choosing it as the target of Stealth, so that I can't use it as one of my defenders.
Keywords do not count as character abilities. Quaith can indeed be bypassed by Stealth or killed by the Deadly keyword. Keywords do count as card effects, though there is a special rule that they cannot be canceled.