Crossposted from cardgamedb, as I couldn't get an answer here. Would appreciate your expert advice, especially regarding example 2:

I find that if a card has "otherwise" in the text box, I am likely to be unsure how to interpret it.

Your thoughts and guidance please:

Examples:

Example 1: Warden of the West interacting with Destitute Horde

Warden of the West attachs to a House card and has Prized 3.
Destitute Horde has "Response: After a Reinforcements event card is played, choose an attachment controlled by an opponent. Take control of that attachment, and attach it to Destitute Horde, if able. Otherwise, discard it from play."

Targ player plays a Reinforcement event, and activates the response to choose Warden of the West.
Does he:
1) Take control of it, then discard it, allowing the Lannister player to claim 3 power?
2) Fail to take control of it, and then discard it, allowing himself to claim 3 power?

Example 2: Gates of Winterfell

Robb Stark is in the dead pile.
Gates of Winterfell reads "Marshalling: Kneel Gates of Winterfell to reveal the top card of your deck. If that card is a House Stark character, put it into play knelt. Otherwise, discard it."

Player kneels gates of winterfell and reveals Robb Stark. Rules state that you can't play or put a unique character into play if it is in the dead pile.
Does the player:
1) Discard Robb Stark because "otherwise" refers to putting into play knelt?
2) Leave Robb Stark on top of the deck but revealed, because "otherwise" refers to if that card is a House Stark character. Which it is.

Example 3: Planky Town Orphan, and choice

Planky Town Orphan reads:
Response: After you play Planky Town Orphan from your hand, reveal the top card of your deck. Play that card as your next action this phase. Otherwise, return it to the top of your deck.

Note it doesn't say "you may play that card", it says "play that card"

Martell player plays Planky Town Orphan. He reveals The Prince's Plans . He has the influence to pay for it but wants the influence for other things and isn't really wanting to play it right now.

Does he then have to play that card, whether he wants to or not?

Generally speaking, "otherwise" is going to kick in if anything before an "if able" fails.

Example 1: Warden of the West interacting with Destitute Horde

The thing to recognize here is that the "take control" and "attach to Destitute Horde" are independent (you can tell by the lack of the word "then" and the extra comma between them). As such, even if one can't happen, the other does. So with Warden of the West, the "take control" part is successful, even though the "attach to" part is not. Because the "take control" part is successful, the Targ player controls the attachment when the "otherwise, discard" effect kicks in - which means the Lannister player collects the power for Prized.

Example 2: Gates of Winterfell

Again, remember that the "otherwise" is going to kick in anything before it is unsuccessful. A successful resolution of Gates of Winterfell would be revealing the card and putting it into play. So if for any reason , the card revealed cannot be put into play, it is discarded - whether it is a Stark character or not. Flipping over a Stark character that is in your dead pile (so it cannot be put into play) is not going to be considered a successful resolution of the second part of the effect, so the "otherwise" kicks in and the card is discarded.

Example 3: Planky Town Orphan, and choice

The text of Planky Town Orphan does not give you the option to refuse to play the revealed card. Like Val, if you can play it, you must play it. (There is no "may" or any other word that implies you can choose not to play it.) By triggering Plankytown Orphan, you are creating a lasting effect that takes away the normal "you don't have to use a triggered effect if you don't want to" choice. So if you get Prince's Plans with the Orphan, if you have the 4 influence, you must use it. The "otherwise" is only going to kick in if you are unable to trigger the card because you can't pay costs, choose targets, etc. But, like Val, the text of Plankytown Orphan does not give you the choice to discard the card instead of playing it.

Thanks as always for excellent and swift answers, ktom.