I always thought it was unlikely that Chapel, as a newly qualified doctor, would be considered suitable to be CMO before McCoy was drafted in. Maybe in light of naval procedures, her appointment would have been considered suitable as a stop-gap considering that the ship was launched before some of its intended crew was assigned (presumably still serving on all those ships that were not in range).
Yes, that sounds plausible. Everyone was scrambling to get the
Enterprise ready to handle the V'Ger crisis, so it's not a stretch to assume that some of the personnel weren't going to be the permanent crew. Medical would probably be one of the very last departments to be staffed up before launch, as you wouldn't need them the way you'd need engineers. [Insert your own GEN "Tuesday" joke here.]
Roddenberry's TMP novelization has a horrible bit in it where, when Chapel learns McCoy is taking over as CMO, she says something like, "I've never been so pleased and relieved about anything in my life." It really reads like Roddenberry's inherent sexism coming through, as it implies that even
Chapel didn't think she was up to the job of being the
Enterprise's CMO. Try to imagine McCoy being told his old boss was taking over Sickbay and him being "pleased and relieved" about it. You can't. He'd be
PISSED.
On the thread topic, I can't imagine them bringing Boyce back unless they found some new twist for the character. He's a bit on the obscure side, and he'd probably just come across as Dr. McCoy Lite if they presented him exactly as he was in "The Cage"/"The Menagerie."