That fly-bys and grazing impacts occur is, of course, a proven fact.

However, the proportion of craters that involve significantly oblique impacts is very low (1% or less), because gravity curves all incoming orbits to be nearer to perpendicular, at the same time as it accelerates bodies towards the impact site (or, strictly, the underlying centre of mass of the parent body).

In very rare circumstances this leads to a near miss, but the bodies that miss are perturbed by their close passage (even not-so-close passages).

What I found laughable in your original post was the idea that the population of orbiting objects would have a Mars-sized-and-shaped hole in them, and that merely building a volcano on the surface (Albeit the largest one in the solar system, and one that essentially sticks out of the atmosphere) would allow new impacts to occur that otherwise would not have.

Were this the truth, the sides of the high volcanoes would be peppered with vast numbers of craters from the newly-intersected population of impactors.

You might be better off to note that since Mars has contracted by 0.3% since the Hesperian (my work, reference here, 1.1 Meg pdf ), then this implies an 0.2% decrease in its cross section and hence a drop incratering flux far more important than any pimple on the face of the planet.

Also, if you think about the consequences of how a volcano grows, lava is squeezed out from the interior to produce the edifice, so the rest of the planet must shrink to compensate for the production of magma, so the result is a zero-sum game. The planet as a whole does not increase in volume or cross-sectional area, and extra volume locally is compensated by less elsewhere.

On several levels, your logic is flawed, your thinking simplistic, and you have missed the most interesting aspects of the situation you yourself proposed.

Why are the flanks of Olympus Mons not peppered with lateral impacts?