In this area I defer to Bill Hartmann who is an experienced and reputable scientist and a good friend.

The issue of dating surfaces on Mars is like hanging a piece of bent wire across the corner of a right angle.

The wire MUST pass through both end points (The origin of Mars, @ some 4.5 Ga, and today, at zero age).

Between those two known points, the wire may bend in many ways and we have very few dates from Mars of any sort to help constrain it. The Handful of Mars meteorites have ages of (a) 130 to 1000 Ma for most of them and (b) one example at 4.5 Ga (ALH84001)

These fixed dates don't help much, since we don't know where on Mars they come from.

What we do know is that there are a range of terranes on Mars with a range of ages, and these have to be strung along the wire like beads.

The final piece of information comes from the Moon where we have (or more precisely Bill has) done similar crater counts and set up a similar relative chronology. For the Moon, we have absolute dates from a huge trawl of Apollo moon rocks, that lets us determine the shape of the curve.

We can also use the few impact ctraters on earth and the current flux of meteors and size ranges of asteroids to help constrain rates vs sizes.

So for Mars, we can assume a similar shape as the Moon, as a starting point, since both bodies are in the same solar system. This give us a reasonable grasp of time on Mars, but isn't by any means absolute.

However, the eroors and the "slack: in the wire are limited and most of the uncertainty is in the mid ranges of Mars time, where the wire can bend most. So for the period 1-3 Ga we have a wide uncertainty in time, but for the periods 0-1 and 3-4.5 Ga there is much less room for manoevre. In particular. there is little slack for the last few hundred Ma, where everything agrees.

Your Phobos analogy fails because all the debris would fall in a limited belt, matching the orbit of Phobos, or would be in patterns of secondary craters that form rays and streaks, pointing back to the primary impact.

There is no evidence ofr a major in-orbit break up of a former Moon of Mars for the last 3 Ga.

other uncertainties are possible, like major debris-forming events in the inner solar system, but they are unlikely. They certainly don't have much effect on the recent few hundred Ma.

Anyway, if you want to know more, you should read Bill's work in the original. He published plenty of abstracts for LPSC and other conferences and has a website of his own at LPL Arizona. He's also a damn good space artist, and I'm lucky enough to own one of his pieces.

Justin Kelly has another - a freehand sketch in the dedication of a volume.