. That's what I thought. The label "executive officer" just wasn't used much in Trek. "First Officer" and "Second Officer" were just used in reference to the chain of command, not to particular duties. So Spock was First Officer, but his duties were those of a Science Officer. His performance as both First Officer and Science Officer should not be any more remarkable than Scott's performance as both Chief Engineering Officer and Second Officer.
So I have to think that Spock isn't retained because he's so remarkable at pulling double duty - all Starfleet officers pull double duty, it seems, or rather none
of them do. They all have duties that they perform, and a position in the chain of command.
Political connections are looking to be the best explanation for Spock's continuing service after hijacking Enterprise