I haven't ever been able to successfully use WD-40 to start a diesel, I know they changed the propellant in it a few years back, but, I've never had luck with it.
As for the after glow, it causes absolutely zero harm to the engine as it's just adding heat to the cylinders to bring up combustion temps and make the engine run clean, not adding a more volatile, easier to ignite fuel like Ether. My Dodge cycles the grid heaters after startup for the same reason.
With my 6.9, below about -30, I just skip the glows and use ether, need all the battery power to spin it over, even plugged in.