The realization I keep coming back to is that a carbon tax is the most fair–and only reasonable–way to let consumers trade off the climate-change impact of one product over another.

Markets don't lie, or at least do so less often than politicians, environmentalists, or corporations. If the cost of carbon emissions were built into the prices of everyday products, we wouldn't have to do any complicated math or what-if scenarios to figure out which product is the best deal for both us and the earth. The best deal would be (all else equal) the cheapest product.