They've been separated before. It took an Iblees nuke to do it.
I really don't see the reason to completely outline every single detail with druidism that could possibly happen. I can understand wanting a bit more clarification on the stones themselves, but visions vary from person to person. The best part of druidism is its flexibility. There's such a wide range of things you can do with druidism because of how its structured. I could write a full-length novel entailing everything you could do and barely scratch the surface because nature is just so varied. Nobody wants to sit through a thousand-page document reading over all the possible things you can do with it. That just robs you of the creativity to come up with something clever. It's a vision. It's flavor. All it would create is character development. OH NO, NOT CHARACTER DEVELOPMENT.
Redlines here would obviously include things like moongazing. That's stomping on other magic's toes, which I do not and never will appreciate. I don't feel they should be exact details either, they should be symbolic and spiritual visions, not Druid used Future Sight! Druid foresaw an attack! Or if it was going to be a sort of glimpse into things to come, it would be very foggy and difficult for the druid to interpret, like a vision of what may be, not what will be. Like if X doesn't happen, here's a vision of what will occur, but not like... explicitly stated. Just tiny images that would be up to the druid to figure out what they mean.
They're unnatural beings. The stones are, in essence, an embodiment of perfect balance created by the very aenguls that created the balance. They're walking corruption, going directly against nature in the case of ghouls as an example. However, I would say that it should only block certain dark magics or certain aspects of dark magics, rather than blanketing over all of them. Necromancy would definitely be blocked from causing corruption or taint within the radius entailed in the post by the OP. Their tethers would also operate at roughly half capacity within that radius, as the stones would be drawn to correct the damage that was being done within the radius. I think at most Frost Witches would feel about the same as ghouls would within the range. I'm not sure how their magic works. Same with Shades, just feel sick around the stones. Although passing on their shade-ness should be more difficult within the range, as again its unnatural. Blood mages would probably be fairly unaffected. Same with Soul Puppeteers, though curses should be weaker within the range. Does not outright stop them from creating the soul connection or inflicting curses. Ummm... Fi magic is technically voidal, so they shouldn't be affected either. Although I think a Fi mage should be able to temporarily stop them from regenerating energy expended, but not outright block them from using the energy they already have pooled inside them. I can't think of any other dark magics off the top of my head.
I figured it'd be like a larger version of a fairy ring. Like... advance by a full tier instead of half a tier though. If already T5, just treat it as if a T2 druid was powersharing with them.
I believe in the OP it was explained what would happen if a stone was moved to the void or the nether, being that they'd dissolve and reform in the real world. I think if they were shattered it could create some very cool RP if the shards then scattered (limit that to a set number that powerful druids would then receive hints as to their location for little events, I don't want to go on some wild goose chase for seven shards or more. If I wanted to do that I'd play Sonic, not LotC) and had to be gathered and brought to the remaining stones to reform and remagnetize. That would prevent them from being lost as well as give the ET a few easy events to do should a stone actually be shattered.
That being said, already stated that it took an Iblees nuke to separate them before, so we're taking direct aengudaemonic influence to destroy them. These are aengul-forged artifacts after all. If they're made by an aengul, I personally don't feel that anything short of another aengul could break them, and that goes for any such artifact.
I would say each would have a stagnant energy pool of two T5 druids apiece for a total of the equivalent of 6 Ariks (Congratulations, Song Druid. You are now a unit of measurement). That being said, I think 48-72 hours would be a fair recharge time assuming all three were entirely drained of their energy. So roughly raid cooldown.
Already answered by Arik, but I'll state it in a bit more detail. No. For reasoning, see aengul-forged artifact argument in the last paragraph. These are directly tied to the Aspects themselves. Even if a corruption was attempted, the inherent nature of the stones would be to rebel against the corruption and purify it. It would create a paradox.
And that's my /2cents