From the bits of dialogue I remember, we don't know that Khan would have performed "missions" for Marcus. Supposedly, Khan provided input on how to build new weapons, or at least how to improve on them. This may well have been a full-time job, but more importantly, it is likely to have been a short-leash one. Marcus wouldn't have much motivation to let Khan wander outside Section 31 think tanks.
At least not until he chose to use Khan as an excuse for starting the war with the Klingons. Did he choose that after Khan rebelled and escaped? Or was the rebellion and escape part of Marcus' plan?
The stardate data seems definite enough. Although the new format is awfully confusing. Robau gives the date as 2233.04, as if the zero served some role there. But evidently the system isn't positional: there's no requirement for the 4 to be the second digit after the dot, obviously. Or else 2259.55 would be the 550th day of the year rather than the 55th. So why the zero? Is Robau just a zero fetishist, insisting on having his starship's pennants repainted, too?