The Wormhole wrote:
Carol Marcus being in Starfleet, and on the Enterprise, is another one of those silly small universe syndrome ideas.
In the Prime universe, it's highly unlikely. She and her whole group were shown to be civilian scientists, and her son had a hatred against anything Starfleet (which certainly would have played out differently when "she was one of them once").
Not necesarily. Even if we assume Prime Carol Marcus was once in Starfleet, we don't know the circumstances for why she left. She could have left under less than pleasnt circumstances and her bitterness towards Starfleet could have rubbed off on David causing him to actively hate Starfleet.
And besides, we don't know that Carol is assigned to the Enterprise, or if she is brought on board as a specialist or something for one mission.
Kirk and Marcus meet in both realities. Kirk is in Starfleet. It is reasonable to assume that perhaps Marcus worked for Starfleet at some point. There has to be a reason they would meet and have enough time to get to know each other.