fossil fuels being literally the most energy-dense per unit-of-infrastructure-applied energy source in the solar system,
Huh? At which level of technology? And WTF is a "unit of infrastructure"?
At any level of technology. Where else in the solar system do you have that much highly reduced matter next to so much highly oxidized gas with a thin layer of rock between them, and something as simple as a drill and a furnace needed to extract the coal energy and a little fractional distillation to get at the oil? Everything else is more difficult.
"Unit of infrastructure" ~= amount of energy and effort and capital needed to get at it.
You know the drill - If it's worth saying, but not worth its own post (even in Discussion), then it goes here.