1.) The story can be viewed as a story from life to death and back in two different ways, the castle can be viewed as a part of the fairy or underground, or gawain's faith can be seen as being tested, killed and reborn.
2.) Sir Gawain is reborn physically because he thought he was dead, he truly believed the green knight was going to cut off his head. By fulfilling his debt to the green knight he is able to be reborn. Spiritually he is reborn because throughout the book he is slowly losing his faith. At the beginning he prays often and worships his shield, however by the end he resorts to magic. After the green knight tells him of his trap, he resorts back to his belief in god and the virgin mary.
3.) You are supposed to feel sorry for Sir Gawain. The author intends the reader to view Sir Gawain as a perfectionist, an honorable knight who is determined to follow his vows. Gawain stands not only as an archtypical knight to help to encourage chivalry and faith among other knights, but more importantly a symbol of determination. He is determined to be a perfect knight, but due to his nature is unable to attain it.
4.) We will not see great deeds from Gawain again, his temper gets in his way. This is his flaw, his frustration. Because of this he will not recover from the embarrassment he felt at the hands of the green knight. He will attempt to carry on and strive for perfection, but will always remember his failings more than his deeds.
5.) The green knight is in control, if Morgana was in control and her plan was to scare/kill lady Guinivere, she would have no reason to release Sir Gawain from his bond. She would have put Sir Gawain to death and hoped his death would have destroyed lady Guinivere.