Your concern is that nothing can change itself...

I don't think the Aristotelian principle is quite that nothing can change itself, in this sense... it is that potentiality is only reduced to actuality by actuality, but the distinction between relevant potentialities and actualities doesn't always map out to a distinction between two distinct bodies. In Aristotelian physics, a given body is sometimes capable of moving itself, as most obviously in the case of animals. But this also happens with inorganic things...

In dealing with the development from Aristotelian physics, I think it's essential to note the important distinction for Aristotle between vertical and horizontal motion in the sublunary sphere. For Aristotle, space is organized around an absolute center, so that the vertical dimension has an absolute structure on the basis of what can be identified from any perspective as an up and a down direction, i.e. away from or toward the absolute center. In the sublunary sphere, inorganic bodies have a natural principle of motion which situates them in this vertical dimension which structures sublunary space. This natural principle of motion Aristotle identifies with heaviness and lightness, so that light things have a natural impetus to move upward and heavy things have a natural impetus to move downward. Aristotle even uses this vocabulary (On the Heavens iv:1:307b32).

Sublunary motion on the horizontal axis is categorically different in Aristotelian mechanics, for sublunary space has no horizontal structure. It's, as it were, indifferent to any natural body where in the horizontal plane it falls. Thus, horizontal motion for Aristotle is unnatural in a way that vertical motion isn't: a natural body will only move horizontally if someone else is pushing it in that direction.

So it's not quite that Aristotle has no notion of impetus. But your point about the significance of relativity enters here also from a different direction. One of the crucial developments toward modern physics is the rejection of the Aristotelian doctrine of an absolute center, without which this whole mechanics falls apart--hence the horror about abandoning geocentrism, which was hardly a mere Biblical prejudice distorting the reasoning of natural philosophers. With geocentrism abandoned, motion has to be rethought--we need some way of defining motion other than with reference to Aristotle's absolute up and down directions. And with this we lose the crucial Aristotelian distinction between the horizontal and the vertical, which refigures how we must think about impetus, and we enter into the series of disputes about referential frames which you introduce.

So, it seems like the OP has introduced the dispute between Aristotelian and Newtonian mechanics as if the problem were that inertia contradicts the Aristotelian formulation of a principle of sufficient reason. But I don't think that that's really where the dispute lies--it's these details of mechanics and of the organization of space that change, not the metaphysical demand for a law of causality.