Acceleration the rate in increment in velocity .A photon doesn't accelerate .The photon velocity can not increase more than c .Deceleration is different .Deceleration is the rate of reduction in velocity a photon can have a reduction in its speed and this true ..A photon will not have any acceleration in any circumstances in the universe .Ah, so you are denying the existence of gravitational lensing. That's good to know.
Pushing a flashlight forward will not increase light speed . So photon has no acceleration or inertia .
pushing a flashlight forward is a good example pushing will not accelerate a photon because a photon has no rest mass.
Mass and inertia are proportional .And both can prevent mass to reach c. Einstein used mass instead of inertia and made the mess in reply #24