Rules FAQ
Each non-Wildling character only loses one instance of the stealth keyword. If a character has multiple instances of stealth (for instance, one from its printed text box and another one from The Brotherhood Without Banners agenda), that character will still have stealth.
The stealth and assault keywords resolve at the same time during challenge initiation. You cannot assault Skirling Pass first and then use stealth afterwards.
Rules FAQ
You can trigger Alys any time she stands, including the standing phase.
If Sigorn cannot gain power (for instance, due to Karhold), the pre-then aspect of the ability has not resolved successfully, so you cannot draw 1 card either. (See The word "Then".)
Rules FAQ
You can trigger the reaction even if it is an opponent who has the Summer plot card revealed.
When power is moved to a character (for instance, by The Starry Sept), that power is not considered to have been gained, so you cannot trigger Godsgrace in such a scenario. (See Moving Power.)
The 1 power the character gains from Godsgrace is independent from the original power that provided the triggering condition in the first place. For instance, if Ser Edmure Tully is in play, he could react to the original power gain, or he could react to the power gain from Godsgrace. They are separate triggering conditions.
Rules FAQ
If you remove the only attacking character from the challenge, the challenge resolution still proceeds as normal. It is still possible for the attacking player to use card abilities like Cohollo or Greywater Watch (JS) to add new attackers.
You can target any character that is currently attacking or defending alone, even if multiple characters were originally declared as attackers or defenders. For instance, if using the ability of Doran Martell (SoD) causes another character to become the only attacker, that character then becomes an eligible target for Nymella's ability.
You can target your own characters with Nymella's ability.
The character you remove from the challenge does not stand.
Rules FAQ