I wasn’t keen on overwriting Chapel, because there’s no need for her to be in the show if they wanted a different character.
I assume the writers are keen to have a character on hand whom we, or at least the audience of TOS, know will eventually develop an unrequited love for Spock.
Lieutenant Galloway. The man so strong even a phaser set to kill and vaporize its target can't keep him from coming back.
David Lee Ross also played Johnson in a different episode. Perhaps the only error is that they call him Galloway in the end credits of the final episode? He is not identified by name in the episode.