I'm going to science the shit out of this one way or another, even if it's metaphysical science.
Sure, the barrel may not effect Eremiel. I don't remember what his reaction was to it. But no matter what, if angels and demons are going to do things like launch arrows of light, control water and summon hellfire, then their attacks MUST have some impact on the physical world. Going with the logic you've presented, demonic attacks should do nothing to the local environment, yet despite this Steven has used his demonic power to kill people and break a water tower. Therefore, at the very least demonic attacks have physical impact. Angels and demons must also possess some level of corporeal form, otherwise they would be unable to screw and impregnate humans, thus ruining the entire basis for the RP.
I'm treating hell and heaven as alternate dimensions of some sort and holy/demonic energy as just that - a form of energy. The kinds of things we've seen our characters do relies on manipulation of matter and energy, such as creating arrows of light and fire. This is meant to be the real world, just with angels and demons. Therefore, science still applies, and though we may need to give a suspension of disbelief for some parts, science should be able to explain this RP.
In the words of Tony Stark; "Magic is just science we don't understand yet."