Yeah I don't think they eat that much, honestly. I have never tried to determine exactly how much food troops consume, but I believe it to be negligible. I mentioned it not because I think it would be significant in practice, but because it's possible in theory.
As far as I know, yes, infiltrators can burn granaries. I doubt one could do it on a sustained basis though, especially if there's also a huge quantity of militia in the region (which would make infiltrator actions harder to complete).
