Doesn’t this just cost them energy and they excrete it? What’s the motivation?
Also how do chemolithotrophs get NADH?
nope, the reaction CO2 + 4 H2 -> CH4 + 2H2O has a deltaG of -170. http://www.fao.org/docrep/W7241E/w7241e0f.htm
On your second question, I would say it depends on the exact organism and the pathway they use