I'm not really a fan of Formula E myself, but they are doing away with car swaps next I believe. My point is that the argument of whether electric cars consume fossil fuels is very dependent on policy and infrastructure and often whereabouts in the world you are. It's as green as you want it to be, within reason, and Formula E demonstrates that for a racing application it's doable. Glycerin is a by-product of bio-diesel manufacture and can be manufactured using natural processes, it is a renewable source.
As for EV vs. ICE on track, we're a way off parity being an issue, and as F1 and WEC can prove, at the moment, a combination of the two is currently the best option*.
*unless you're driving a Red Bull.