Lloyd said:
In which proton repulsion experiment/s were photons not detected? Did any experiment ever look for photons as the cause of repulsion?

Actually, I think that people have been looking for this ever since Eddington first proposed it in the 1920s as the "force" that prevents the gravitational collapse of stars. But that's not the point. If the Coulomb force reduces to photonic pressure, that's a lot of force. The scattered photons should be easy to detect. We have a good idea of how much energy has to be converted to get photons, and that's the same amount of energy that gets absorbed when they land on something. If that same energy is responsible for the Coulomb force, I'd expect the force between the two terminals on a car battery to be blindingly bright. But alas, the Coulomb force doesn't glow in the dark, in visible wavelengths, or in any other wavelengths.