Arthur should not forgive Lancelot due to his disloyalty because it greatly affected him and his life. His wife, Guinevere, was having an affair with Lancelot and it took a toll on him. Arthur should only forgive Lancelot if Lancelot earned his trust back once again.
Sir Gawain is loyal to King Arthur because he proved that he can be loyal and chivalrous when no one stood up against the Green Knight. He stood up against the Green Knight despite the fact that he was still not a knight.
Yes, loyalty can be restored even if it was once lost. It can be restored as long as the person was willing to trust the person who betrayed him once again and if the person who betrayed him earned back his trust.
It intensifies the conflict.
A complication adds to the problem of the story. The stories problem is the conflict. Many times there is more than one complication. When labeling a plot line for a story, the complications can be found in the rising action. This takes place between the exposition and the climax. The exposition of a story introduces the characters, setting, and conflict. The climax is the pivotal point in the conflict. The character can never return to the way things were when the story started out. Something or someone has been irrevocably changed.