I have joined in this very late. I tried to read all of the above, but ...
This is what I understand.
A photon has mass, but not real mass, only effective mass. In reality, the effective mass is due to its momentum = E/c.
Here is what Wigner argured. If a photon has mass, you can acclerate it or de-accelerate it. But you can't. Poor photon is stuck with its c. (Slowing down of light in water is something else; don't be confused here.)
Similar logic with its spin. Its spin = 1, but no zero polarization! (A particle with a mass and spin 1 has three polarizations: +1, 0, -1.)
What do you think?
I have introduced a new topic: Gravity's effects. Can someone answer me there.