Yes, more stuff means more scattering, but scattering happens at all scales.

Photons don't cause absorption or scattering with themselves, no, but they do interfere. The interference really only shows up when the waves are nearly in-phase, so random light rarely interferes, but it should happen.

Photons aren't matter anyway, so I can't make a substance out of them. The rest of the bosons should interact with light even less, although the gravity of their energy might bend it the tiniest amount.