I don't know. I think the fact that we've learned that Alfred can release parts of prisoners is potentially relevant to this conversation.
Look, everyone knows I'm a big fan of 'Demonreach is fueled by archangelic Grace'. But if not, then there has to be a source of magical energy to fuel Alfred's wards and defenses, right?
So is it theoretically possible that Alfred is 'releasing' a percentage of each inmate's magical power, and Alfred is distilling it into a non-evil source of energy that he can use to power the prison and its defenses? And the leftover energy (likely the bulk of it because the extraction process likely isn't efficient) is what's creating the dark ley line?
If that's so, then couldn't there theoretically be a scenario where Demonreach creates a tap for Harry? It'd release a ton of dark energy into the ley line, but allow him to tap a pure source of energy to use? Either for a massive spell, or a Darkhallow, or something along those lines?
I'm not saying it's likely, but it seems like it's not outside the realm of possibility.