• the substrate is space itself. Velocity is defined as the distance an object travels through space over time.

-Sorry I think I was a bit unclear - 'True' mass is an intrinsic property of the object, so that doesn't change. What does change is the energy of the object, and there is a law for equating energy and momentum with velocity. The momentum increases as the kinetic energy of the object increases, so this can be thought of as kinda like an increase in 'inertial' mass. Something which is also important is that an object gains momentum only when it is accelerating. If it's moving at constant velocity, there is no net force so its energy doesnt change.

• I don't really understand what you mean by time dilation cancelling out. Time dilation occurs when you are moving through space at a given velocity. We can observe objects from inertial reference frames, such as the Earth, since the Earth is moving at a constant velocity, so we can assume that our time dilation is 0, and then find the time dilation of an astronaut relative to us that is moving through space at faster velocity.

Remeber, this phenomenon only occurs becuase light speed is constant regardless of what inertial reference frame you are in. It doesn't matter if you are moving at 0.9c and then shine a light, or if you are stationary and shine a light, that light will move at the same speed. the theory of relativity basically makes rules to account for this phenomenon. If you take a look at the Lorentz Factor (the one that you calculate time dilation with) it's actually very easy to derive, you basically take two scenarios where light is travelling in a moving object and a stationary object, and then you state that the light has moved the same distance in both cases, even though that sounds ridiculous.