Expansion of spacetime stretches wavelengths and produces the red shift. Does it also stretch the amplitude of the wave, and make distant stars look brighter and therefor nearer?
The intensity of an EM wave is proportional to the square of the amplitude. I think you're getting confused here by wave-particle duality. The wave stuff doesn't stop being true just because it's also a particle. If you consider a single photon, it has electric and magnetic fields, and the energy density is proportional to their squares.Amplitude has nothing to do with brightness in an EM wave. Intensity of a light source is dependent on how many photons you are able to collect.