Infinite universe: Thought that this was pretty settled science? Or at least that it's much bigger than hubble limit? Why must entire lightcone leading to Solar System have only one intelligence? Are you assuming that all intelligences will singularity faster than geological time, and then intrusively colonize space at speed of light, thus preventing future intelligences from rising? What about intelligences that are really, really far away? I think you are making really unjustifyable assumptions. I think this kind of anthropic stuff is... risky.

Would we be able to see a bronze-age civilization 500 ly away? Possible that such things could be more stable than ours? And a bronze age civilization is pretty different from nothing, more like ours than nothing.