I was recently reading Walt Cunningham’s book (LMP on the Earth-orbiting Apollo 7), The All-American Boys, and he said Deke Slayton had decided that only those with test pilot experience would serve as Command Module Pilots, although Slayton kept this to himself at the time. Meaning that the absence of that qualification meant you were more likely to walk on the Moon. CMP was considered a more important job than LMP – as noted above, he was alone, he had to fly the Command Module during rendevous (while the mission’s Commander would be at the helm of the LEM).
Oh, also, in addition to Young and Cernan, Dick Gordon, CMP on Apollo 12, was scheduled to command Apollo 18, but the lunar program was canceled at 17. (With Gordon’s LMP, geologist Jack Schmidt, taking the place of Joe Engle – Engle finally made it into orbit on the Shuttle in '81.)
On Apollo 10, when they separated the lunar lander from the command module to test things, did that make John Young the first person to be as isolated as Collins is attributed?
I don’t know the answer, but I expect that, unlike on 11, the LEM and the Command Module were always at least within radio contact of each other. Collins’ total communications blackouts came when he was behind the moon while both the Earth and the Eagle (having landed) were on the other side of it. Since the Snoopy (10’s LEM) never landed, I expect its orbit and the Charlie Brown’s were close enough so they didn’t lose communication.
I once saw an interview with Collins in which he said he was just honored to have been selected to serve on the Apollo 11 mission. While in orbit he felt at peace and savored his solitude. He ran some experiments and took photos, and was never bored. He had some alcohol problems after returning to Earth, IIRC, but I have no reason to believe that was related to not having walked on the Moon.