I start by looking at the overall history of the Althans, as you note about30,000 years - a longer span than the whole of human history. I've dived in and created several ages of their civilization, also stretching the history to about 37K years.
The next thing I note is that the interstice itself is not typically the case of all of Kulthea's woes. It creates a baseline for allowing magic and portals, sure, but it's Great Sa'Kain that really brings the catastrophe. We have the year of the first coming of Sa'Kain: 14,000 years after the dawn of Althan civilization. With a periodicity of about 1,500 years we should expect to have seen it nine times before. Either some sort of cataclysm happened to send Sa'Kain moving or Sa'Kain is an engineered phenomenon.
Either way, 16,000 years is more than enough time for Althans to have developed from the stone age to the fusion age. And all of that would be knocked to rubble when the comet came and suddenly there were demons. Think about would would happen on Earth today if we suddenly had demons coming out of portals on land, sky, and water. And we are not where the Althans would be -- 16K years ago our big achievement was pet dogs.
So Althans have to start from near scratch once Sa'Kain passes. This would have been the first period of serious magical experimentation, too, all that essence in the air gives opportunities to experiment that hadn't been there before. But 1,500 isn't that long of a time when starting a technology from nothing, so they'd have still beenn in something of a magical stone age when Sa'Kain returned. It would still have been catastrophic, but increased understanding would mitigate matters.
Eventually, if the decision is made to fight them on Althan terms instead of demonic turns. Thus, roughly mid-way through the whole of Althan existence, they start exploring the Pales and sending ships into space past their solar system.
Long story short, this new society has factions and politics and motion, and every 1,500 years they're better at handling Sa'Kain using the entire resources of their empire - including treaties with demonkind that form the basis of demonic magic in Game-present-time. One of things that happen is that Althans start breeding for psions and spellcasters, which eventually produces K'ta'viir.
Which brings us back to your question of why Kulthea didn't fall into the breach one way or the other. One reason is The Lords of Orhan. I posit that another reason is that the world falling into one universe or another would be no more natural an occurrence than Great Sa'Kain, and the powers that want that derived more benefit from the interstice remaining open.
So why the eyes? Because as the Althans rose, their ability to dictate terms rose, with Kadaena representing a zenith of power. The sudden loss of the Althan empire means that demons who were being exploited suddenly have free access the existing control mechanisms off the table. The Eyes are a new control mechanism, perhaps one aligned to Utha's philosophies on compelling labor from demons rather than Kadaena's. Onndoval is confident that he can safely close the eyes because he sees himself as Kadaena's successor to the throne and believes that the Shadowstone will give him the full power it gave her.