POD: Robert A. Heinlein's first and short-lived marriage never happens.
OTL he was closing in on a Rhodes scholarship. Since those were for single men only, he had to withdraw. TTL he doesn't withdraw, and gets it. (That's not absolutely certain, but Heinlein was near the top of his class at Annapolis, so it seems plausible.)
So. Two years at Oxford for young Robert. 1929-1931; he'll be 22, 23 years old.
...autumn 1929. C.S. Lewis, a junior professor, was just converting "kicking and screaming" to Christianity. The following spring J.R.R. Tolkein, grading a bunch of exam papers, would turn one over and write on the back, "In a hole in the ground there lived a hobbit."
He'd meet other Rhodes scholars there. Who? Well, Robert Penn Warren one class above, and E.F. Schumacher one class below. And other undergrads -- he'd overlap with Alan Turing.
Let's say that his Navy career is otherwise unaffected; he still graduates with his class, still contracts TB a few years later and is discharged.
Now what?
Doug M.