1) A multiplayer title's challenge bonus (eg: Crown Regent's +3 to power challenges) is a constant effect that lasts for the breadth of the challenge phase?
=self answer: Yes.
specific card questions:
2) I'm You Writ Small (Lanni event, CS): the boon attachment kicks in following round or on the current phase the deadly effect is still ongoing?
=self answer: it kicks in on the current phase, because of the word "THEN" is not a timing statement requiring the current phase to be over.
3) Condemned by the Realm (Targ plot, CS) what does it mean that your opponent must choose and kill a char he does not control? What does that mean in a two player game?
=self answer: it grants your chosen opponent an instant kill effect that can target any characters (i.e. including your own character) except ones under his/her control.
4) Greatjon Umber (Stark char, CS): can use his ability to join attackers after defenders are declared, thus suddenly boosting the total attack strength by 3?
=self answer: yes.
5) Daario Naharis (Targ char, CS): does his ability kicks in before gold + strength is counted for power claim in the Dominance phase?
= self answer: yes.
Are my assumptions correct?