A "Mars sized hole" is a kind of simple way of considering accretion. What you have is a series through time of possible intersects of moving debris and moving planets, a set which is pretty well exhausted by this late stage, but unfortunately not completely exhausted.

Given this set of intersects through time, Mars shrinkage should result in a very small drop off in impact rate, but doing this conversion would require a detailed set of calculations.

Oblique impacts and fly-bys are rare. Two of the interesting things about these on Mars is that due to the clathrate, Mars' low gravity, and rare atmosphere, low fly-bys produce straight , long, deep valleys; due to the lack of tectonics and water based geology these are preserved, even though rare. Not only fly-by's will produce these features, but comets on a sufficiently oblique trajectory will, as in this case they will vaporize completely in their filght through Mars atmopshere, leaving nothing to impact.

Rare as they are, oblique impacts probably play a large role in the ejection of material from planet's surfaces into free space.

Let us examine the case of the ejection of volcanic rocks from Mars. Most Mars impacts result in destroyed material
rather than ejection. Only material capable of standing the impact force intact, neither vaporized nor pulverized, and struck in a certain direction with a force capable of producing escape velocity, become candidates for recovery here on Earth. This will favor volcanic rock.

Further, the ejected material then has to survive Earth entry, and given what we can reasonably infer about rock types on Mars, most Mars planetary ejecta would still not have sufficient strenth to withstand this, again favoring volcanic rock.

Given the rarity of of oblique impact, the sides of Olympus should not be "peppered" with long straight scars. How many does it have, if any? I don't know - stating the obvious, it's not my field, further, compared to the consequences of Earth impact, it's not that important. Doing that examination is something for someone else.

For you, what is more important here than the ejection of volcanic rock from Mars is that large impacts will release large amounts of both CO2 and water from Mars' clathrates. This will occur periodically, on a 26 million year basis, as the gravitational effects on our Solar System during its passage through the plane of the Milky Way will change Mars position with relation to the Sun and to the debris field in a very small way, but one sufficient to pretty much insure (its chaotic) its intersection with large impactors.

EP