He has to inform congress if he reopens the can of worms, I believe (that is what I've read from a bunch of sources but I can't be sure). So within that universe, his choices are to either sit until the election (the non-interventionist choice) or to look into it the week before (the shit storm choice). Both are morally and legally defensible. Tipping the balance for Comey were the internal and external pressures, including losing his job in shame if he didn't pick the preferred option of the Republicans after he dared go against politics and failed to prosecute Clinton. It makes perfect sense when looked at as a face-saving move.