Software consultants are one counterexample, because they are very expensive but also environmentally friendly. Human labor has the biggest difference between price and resource consumption that I can think of.
No it is not. For example a meat eating human produces more CO2 per mile than a regular car. (Although most people could use the exercise so don't use that info in isolation.)
Again, many people need exercise so this might just replace that. But if you exert extra, then you need extra food. Food costs CO2, and humans are not very efficient.