The mana level shouldn't be a problem as it shouldn't be high enough for a thousand years.
It's already been established that the Invae (Insect Spirits) are already here, and way, WAY ahead of schedule (several centuries, at least).
That wasn't even 50 years into the Sixth Age, and we know the Invae weren't just hopping the spike points created by the Ghost Dance.
Logically, I assume the Scourge Armageddon Clock has moved up and accelerated considerably. The Sixth World is already a miserable, unstable place, and likely doesn't even have 1000 years before the Scourge begins. SRR suggests this is due to the sheer volume of people propping up the manasphere vs back in the Fourth World.
Given how the Immortal Elite Squad is treating the bridge (read: scared shitless), it's almost certainly the main point of entry for the Horrors to cross into regular metaplanar space, and from there, into the prime material plane. We also can deduce that the size of the "gap" as a whole is tied to mana levels on the prime material plane, and mana levels are only rising.
So, knowing all this, I suspect the Fovae are just "probes" of Horror design, to get some sucker metahumans Marked to help accelerate the main bridging process.
As Elijah demonstrated, simply making contact with them is enough to have a nasty effect, and they're quick to offer all sorts of things in exchange for helping them.