OK I feel bad...but here are the answers to you're questions
Yes Arthur is worried and they will save Gwen
Arthur has been dead 14 years and Merlin stayed at the lake,
Arthur was brought back because Camelot needed him.
Morgana was never truly dead, no Mordred did not return,
Gaius came across Gwaine on his way to Camelot and saved him with magic.
Arwen's future remains to be seen and all I can say about Adalia is that she will be a total daddies girl.
thanks for the reviews!