All true, but there was a little more even than that. Charles Williams was a very magnetic and charismatic personality, and more people than Tolkien seemed to think he cast something of a "spell" over Lewis (speaking metaphorically, of course). Lewis was certainly obsessed with Williams from the time he came to Oxford to the time he died in 1945. It not only disrupted Lewis & Tolkien's older friendship, as you point out, but Williams' theology was somewhat sketchy. In his earlier years he'd dabbled in the occult and psuedo-gnostic branches of Christianity, and still carried an air of the esoteric about him - something we old-line Catholics are somewhat suspicious of. Also, if Lewis was too blunt and unsubtle in his writing, Williams was the opposite - obscure to the point that it baffled even T.S. Eliot. Tolkien couldn't make heads or tails of it and, in the manner of the Inklings, said so.Son of Aslan said:Indeed, that what I have always gathered. Tolkien did not think the bond of friendship was strengthened by the introduction of a third person with the same interests, as Lewis did. Tolkien did not like allegory (even in the sense that Lewis implemented it in CoN) and hated the amalgamation of mythologies present in CoN. Tolkien thought Lewis was sacrificing too much in his marriage to Joy. Since the initial marriage was civil only, he thought it silly to marry a woman for charitable reasons. I often wonder what Tolkien thought when Jack and Joy had another religious ceremony where Peter Bide joined them in the eyes of God.
The question of marriage, as you point out, was probably the real "tipper". Tolkien and Lewis had already disagreed sharply about a prospect that Lewis raised in passing in Mere Christianity - that for marriage, perhaps there needed to be two standards and two ceremonies: a civil, which was only understood at the level of human law, and a Christian, which would reflect the high ideals for marriage described by Christ. Tolkien, being trained in Natural Law, said not so: if God said marriage should be a certain way, then all human law should reflect it, whether applied to Christians or unbelievers. Not only did Lewis actually go through with his idea in marrying Joy Davidman in a civil ceremony, but it was to a divorceé - who Tolkien considered still married to her prior husband. This pretty well sealed it between them, and when Lewis got the job at Cambridge, the didn't see much of each other after that.