Upriver it seems you are asking what is the cause of inertial mass, since inertial mass itself is defined and measured by its inertia, or resistance to a change in motion: it takes a force to accelerate it, and the measured amount of force ‘defines‘ both the ‘mass‘ and its ‘inertia‘ (without really defining either). It seems to me there has to be an opposition of forces, rather than a force accelerating a kilogram of ‘stuff‘. You would think that the ‘inertial’ force opposing the accelerating force would be gravitational force, which causes an acceleration toward a common gravitational center. So then you have look at gravitational mass, which again is defined only by a measured ’effect’ between bodies. Inertial mass and gravitational mass are considered to be indistinguishable I think. Once again you are faced with the problem of ‘stuff‘. If mass ‘contains’ a measurable amount of energy, you find that the ‘stuff’ that resists acceleration is actually a container, or form, of energy. So one form of energy - the ‘stuff’ at rest or in ‘constant‘ motion - resists the action of another form of energy - either a collisional force created by other ‘stuff’ already accelerated, or a non-collisional force emanating from other ‘stuff’, e.g. magnetism.

Then what is energy? If it is indeed the ‘property’, or the quantity of the ‘property’, of changing the state of a system, then ‘mass’ itself is some ‘thing’ that is changing. Eventually this ‘property‘ has to be expressed as a force, causing a change in motion (speed or direction), and any ‘mass‘ is itself the effect of a force - not a gravitational force, which governs the behavior of ’masses‘ toward each other, but a more fundamental force governing the behavior of an isolated body of ‘matter‘.

If ‘space’, the ‘ether’, and the ‘vacuum’ are thought of as identifying the same system (perhaps a field), and it is a dynamic system, I believe it is also a fundamental system out of which ‘mass’ is created - a field of force. I believe the idea of rest mass, and then of inertial mass in general (and following that, gravitational mass), is a kind of misnomer, in that no body of ‘matter’ is ever at rest - it, or its components, rotate. So (in this scenario) mass is angular momentum, and what is resisting a change in its motion is already rotating itself. Angular momentum represents a continual acceleration which must be produced by a force. So what is accelerated? What if the ‘stuff’ (as in the 50-kilogram bar in Paris, which is losing weight) isn’t ‘stuff’ at all, but rotational motion? Then inertia is the resistance of an energetic system (undergoing continual angular acceleration by the field) to other forces. So the ‘mechanical cause’ of inertia in this scenario is rotation. (This is opposite to what seems to be the mainstream view, that rotation of large bodies is an effect of gravitational motion.) And the field and its creation, ‘mass’, are different ‘forms’ of energy, which is not a measurable ‘property’ but a fundamental ability - the ability to move.