@Harriet1237, I think you are underreacting and your P is treating you like a fool.
He has clearly had an agenda to block transparency about this woman and that night. He lied by omission regarding the room/bed sharing until you rumbled him, and has actively lied about the lack of clothing and about being with his friend. The lost key story is also suspect. Covering up his boundary trampling has been his priority.
…it just so happened that they were in those places on the bed talking and then just slept…
Really? That sounds like they just leaned back in their clothes and went to sleep…. but that’s not true. When and where did they strip off their clothes? Were they already disrobed while sitting around talking, with her topless??
He is bamboozling you, @Harriet1237. He willingly paired off and undressed with this OW, and they slept together. He said yes to all of that when he had other options, and then he began the subterfuge.
There is much more to this and I would be getting to the bottom of it. I believe there was some physical involvement that night, hence his jumpiness, lies, and drip feeds. Also, he agreed that he wouldn’t speak to OW going forward, but he has. Even if it was only about work, he broke NC and didn’t inform you — another secret. There’s certainly a good chance that he is editing the messages and interacting with her in person at work.
In my marriage, lying and faithless behavior are dealbreakers. Are they yours, @Harriet1237?