Why is c the speed limit?

Is it because the mass of a particle and its waviness increases as it
approaches c?

Is Cerenkov radiation ever faster than c? It's just faster than light
when light is slowed, nearer to c.

Two point light sources coincidentally start emission at the same time
towards each other. Does not each photon hurtle at c thus that their
relative velocity is 2c? The sources disappear and the photons
remain, why is not one at rest and the other moving at 2c? The
visible universe has many light sources.

How would you define a tachyon, and what are its properties?

http://scienceworld.wolfram.com/physics/Tachyon.html

It's kind of like c is 1, and massy particle obeying Newton's inertia
law speed is infinitesimal, and tachyon speed infinite, and particle
and tachyon speed are reciprocal.

Regards,

Ross F.