King Arthur's Curious Role

As I read, I found it curious that King Arthur himself did not apprehend Lancelot and the Queen. As it was, King Arthur "rode on hunting" while fourteen knights hid themselves in a chamber to await Lancelot's arrival at the Queen's rooms. Arthur knew of Lancelot's might, as he warns Sir Agravain and Sir Mordred that "ye shall find him wight (strong)." While it is understandable that Arthur would think that fourteen knights could safely capture Lancelot, it is odd yet that Arthur did not attend the burning of his wife. We know he did not attend because it is commented later that when King Arthur "heard of the death of his noble knights...he swooned for very pure sorrow." Based on my limited knowledge of the times, I would believe that a king would be present at the execution of a subject, especially one accused of treason. Presented in this light, King Arthur appears to be somewhat lacking in the traits traditionally associated with him, such as courage and daring. This image made a striking contrast to the Arthur we read about on the battlefield in the fight against Sir Mordred. In the final scene of this battle, when Modred stands alone, Arthur takes the situation into his own hands by killing Modred with a spear. While reading this, I found it hard to put together in my mind that this Arthur was the same as the King who left the disciplining of his wife to his knights.
How did King Arthur know that Sir Bedivere was lying about throwing Excalibur into the water? Who were the queen and ladies who were on the barge that took King Arthur to Avilion?