What if Aziraphale and Crowley really are immune to hellfire and holy water respectively, at least to an extent? Crowley has been shown to have a heart and he didn't really mean to Fall. And Aziraphale isn't completely angelic himself either. Or, as cheesy, as it is, their love for each other is what made them immune. Of course, we'll never know due to the body swap.

That’d be kinda hilarious that they came up with this genius plan for nothing, but that also sounds like the kind of mistake they’ll totally make