I really don't think it's unusual two priests would spend a Friday night together. These are (supposed to be) celibate men who live with other men, so that becomes their social circle.

However, I just had a new theory spark about the two and the letter. Let's say Koob and McKeon are lovers. The spend the night together, and even are back at Koob's place drinking that tia maria, when the call comes. The next day McKeon panics and thinks he should say he was alone. Koob later convinces him they have to stick with their public actions that night or someone will discover a hole.

As the police investigate Koob's life, both are becoming increasingly nervous. It's one thing for priests to be found uncelibate, but at that time homosexuality was even more taboo. McKeon writes the letter to try and throw off the police if it gets to the point Koob's belongings are searched. Koob was always ambivalent about this, not wanting to tarnish the memory of Cathy, but also not wanting McKeon's reputation to be ruined. Over the years he's waffled on his relationship with Cathy, the story always a bit different. Koob and McKeon do sound rehearsed in their alibi, because they scrubbed out all the parts of the night where they were intimate with each other.

Koob strikes many people as insincere, including the investigative journalist and part of The Keeper's audience, because he has been lying all these years ... just not about what we think.

(This is the probably the least tortured theory I've come up with yet for the letter (I just can't believe Cathy wrote it), though honestly I think Koob was more involved than this story suggests. So I'll keep thinking.)