Heres my take on that -
1- If the Admiral's sole job had been to conduct negotiations, and said negotiations had gone sour, why is his ship still in system? He had plenty of time to leave before the end of the campaign.
2- If he did in fact stay on as an escort, why is he operating with such a small force? You dont send Destroyers into theaters of war with a minimal escort, no matter how powerful they may be. They are simply too valuable. Therefore, IMO, its more than safe to assume that the remaining battlegroup elements are present in Sol, and simply were not in Sunglare. It would make little sense otherwise.
And 3- We come back to my original point. Why send a whole damn Destroyer battlegroup to cover a few logistics vessels that will, ideally, never be near the front lines, and have already proven to be near impossible to find once they have begun operations? Simple - you don't. The only reason I can see for those ships to remain in system is as an offensive force.