Michelle Fairley could say no to playing Lady Stoneheart, but she's said the part would be an interesting challenge to play and that she'd relish taking it on. If Stoneheart is in I think Fairley will play her. My guess is that Stoneheart will be included and that there'll be a slow build-up to a full reveal at the end of the fifth season when Stoneheart confronts Brienne, so Fairley would only be needed for a cameo in the fifth season with an expanded part in the sixth season. I think they'll try to keep her inclusion a surprise, with Fairley secreted in to film her appearance. As for the Ironborn, I think a limited Kingsmoot and the introduction of Euron will be included, while Vic and his voyage to Meereen will be cut.