a. They did an altitude increase. There's no real gains from an earlier burn that one might get when doing something like a hohmann transfer.
b. Half an orbit before out means more/better telemetry to work with.
c. They are from a quick google, well within the range where atmospheric drag plays a role, and you need to do station keeping. leaving it this late means you don't "waste" fuel from drag.
Admittedly, my only credentials for this are a few thousand hours in KSP and reading around the subject. Anyone with a proper degree on this is welcome to correct me.
If that wasn't the case, they could raise or lower the altitude slightly several orbits before the "event" causing them to arrive at the intersection point earlier or later. For a desired time delta, the sooner your perform your altitude change the smaller the changed required.
Basically ESA nicely asked SpaceX to maneuver their sat and shorten its lifetime - they don't have a legal basis or enforcement mechanism anyway. SpaceX pretended not to hear until it was too late and ESA went ahead with their contingency plan of burning their own sat's fuel and lifetime instead.