"How responsible is Arthur for allowing the spread of Christianity and the ultimate disappearance of Avalon? Was he simply being an honorable husband to Gwenhwyfar? Did you find the Arthur/Lancelet/Gwenhwyfar tryst disturbing? Although Arthur was an indisputably potent leader, can he, in the end, be deemed an effective one?"
Arthur is completely responsible for the ultimate disappearance of Avalon. He made an oath to protect it, and he broke it. He had the power, and he didn't use it.
He had the ability, as well as the obligation, to protect the children of Avalon, but he abandoned them in favor of the cross. For all Arthur's talk about how symbols are only needed by and valuable to commoners, Gwenhwyfar was obsessed with removing the dragon banner from use.
The breaking of his oath can't be justified by his duty to be an honorable husband. Being married isn't about doing whatever your spouse wants you to do. It's about coming to mutual agreements. Arthur's duty to the country is to do what's best for it- even at his own expense. There's a reason for the archetype of the Sacrificial King.
I didn't find the tryst disturbing. I found it shocking and hilarious. I don't know why I thought it was funny, but I did.
Arthur protected the kingdom from the Saxons, but he didn't actually do anything. He was reacting to the threat- there wasn't any action involved. It's not that he encouraged the assimilation of the Goddess into Christianity. It's just that he didn't do anything to stop it. He didn't do much of anything at all.
The kingdom found peace under his reign, and everyone loved him, but he didn't do much, so no, he isn't an effective leader.