We find difficulties with the constancy of the speed of light and yet if unaffected by external forces all objects will maintain a constant speed. The vector direction doesn't change. So the natural state of objects where forces are absent is constant speed. So there should be no mystery. In the case of inertia this will increase in direct proportion to the increase in relativistic mass. Which is due to a change in the magnitude of the vector of the objects velocity. This does not necessarily apply to a change in vector direction alone.

What you say is always interesting but often your language is difficult to understand for my engineering mind. The equations I show are my best estimate for the graphs in my 50 year old physics book which I no longer have. So my inertial mass is really an equivalent mass which I believe my equations represent to a close degree.

Who is the we who finds problems with the constant speed of light? As I see it light jumps from space to space. The jump speed of light is constant. As light jumps from one gravitational field line to another, there is a slight time delay. thus the average light speed is less than C. In low gravitational field areas of space the light speed with be the highest which I call the ideal light speed. Within a black hole the jump speed may be C but the density of the gravitational field lines means the light keeps stopping. Thus the light speed could be reduced to near zero.

A moving object at speed v becomes like a photon whose light speed has been reduced to V. thus it will continue to move in a straight line at the constant speed V until it is operated upon by a force which turns it. However if it enters a higher density gravitational field which is balanced on all perpendicular directions, the object will slow even though no net force is applied. Thus the inertial law is only truly valid for linear space time.