1) Does King's Landing allow me to marshal locations from my discard pile into shadows? Same for Night Gathers and Tarle the Thrice-Drowned. Do faction-specific discounts apply to marshaling cards into shadows from discard/dead pile? (since the cards I marshal into shadows are in a visible game area.) Or do I take a card from a visible zone and show it facedown in step 0?
No. “Marshaling a card into shadows” is not the same as “marshaling a location” or “marshaling a character.” Basically, any effect that allows you to marshal a certain type of card or a card with a certain characteristic will not allow you to marshal a card into shadows.
2) My opponent controls King's Landing, and he marshals a location from his discard pile. I use Sea ***** to take control of King's Landing. Can I now marshal a location from my discard pile? I know limits and maxima are player-specific, but this card has neither.
Yes. The “once per marshaling phase” on King’s Landing is player-specific.
[Danny Schaefer, 04.09.2018]