Maybe the number can be decreased not by sealing them away, but by enforcing the opposite of the usual rules for exaltation?
The flaw here is that this whole idea gives the Sidereals way too much personal power over the other Exalted, because even the Jade Prison is considered a one-off event with a deliberately chosen target. Making them the masterminds behind
two such events simultaneously stretches the premise of the Usurpation dangerously thin.
If you really want to displace the Lunars from the setting temporarily, you want to explore two facets: 1) Lunars were not technically made by a Celestial Incarna, in many cases Luna is classically attributed as being an
honorary member among their number who defies similar origins, 2) Lunars spent the post-Usurpation rebuilding themselves and their Exaltations
by hand, which makes things necessary to break it further to make it more of an arduous climb. So let us imagine, fundamentally, that Lunars did not work nearly as close to how they do now, due to a combination of the changes made to the world resulting from the fall of the Primordial Host, and the idea that Luna's strange nature is reflected in them.
It could be Lunars were never intended to Exalt in Creation at all, but because there wasn't anything but Creation at the time, this never became an issue. Humanity bore Exaltation, this was all anyone ever understood because there was never a need to define what humanity Was. The Underworld was born with the first casualty of the War, breaking the wheel of reincarnation, and also explicitly marked its ending. If any Lunars had disappeared in the last of the fighting, it was assumed they had simply abandoned the long-fought battle in the pursuit of their own individualistic goals, as-is their wont.
But the truth of the matter was Lunar Exaltations discovered the newer and darker world below incidentally, by following their dead foes like hunting hounds, Exalting the remnants of the first ghosts to wander the barren wastes, a new and unexpected sub-group of the human condition never before seen. As these Lunars crawled their way out of Lethe and set about conquering the then-masterless Underworld, their living kin aided the Exalted host in retaking and rebuilding Creation, more aging and dying Lunars filtering away into ghosts over time unbeknownst to any until the two growing kingdoms finally met across the barriers of the first shadowland. This was when the First Realm was technically born, as a united compact between the Solar
and Lunar Deliberatives, the living and dead Exalted and humanity.
So for a time, there was a measure of peace, and Solars and Lunars exchanged notes over different rulership policies between the living and the dead, some choosing to reside in Creation and enjoy the Mandate of Heaven, as others sought to plumb the depths of the Underworld for new adventures and exploit the changed nature of their old enemies. Then the Usurpation came, and with it the destruction of both the resident Lunar host alongside their Solar spouses, and the infrastructure of the First Age which allowed free passage from beyond the veil of death without tearing a wound in the world. The Lunars who died again were now marooned alongside the Underworld Solars who had never taken part in the feast, and the Lunars who fled were now on a time limit, scattered and disorganized by the fighting.
As the Shogunate advanced, more Lunars sank back to the Underworld as their Solar partners died and reawoke in Creation at the spear-tips of the Wyld Hunt, only able to reenter Creation at the fringes where networks of artificial shadowlands still held chaos at bay. This is where the Lunar Exaltation began to break down and untwist itself, before finally developing the Moonsilver tattoos to paradoxically ground themselves to reality and the living where chaos and death warred with eachother. But while successful, the practice still placed them far-afield from their seats of power on the Stygian Isle, and from footslogging across the world to meet their enemies at the foot of the Imperial mountain. Only where Shogunate pogroms decimated the servant races of the old Realm did the Lunars find their footholds, battling the Dragonblooded through unclosable gaps they had torn attempting to cement their victory.
The Balorian Crusade and Great Contagion became an unexpected windfall in this regard, spreading intertwined chaos and death across Creation the likes of which had not been seen since the War properly. The Lunars took the initiative in the fighting, and... began to Exalt in living people once more, now faced with a world-ending cataclysm to endure. Not always, but sometimes, and that sometimes was enough to hold back the tide before the Realm Defense Grid rocked the foundations of Creation and drove back the Fair Folk into the wyld again. The dead flooded into the Underworld, staunching it with idle souls, and the Lunar host found itself off-footing once more as they were pushed out and into a Creation which lay equally barren and in need of heroes.
And THAT is where the story of the Second Age begins, with the reborn Lunars on leveled footing against the Scarlet Empire, still coming to grips with reforging themselves through adversity into Creation-based Exalted and existing wholly as the other Celestials do, watching Solars reemerge into the world. The Underworld is in flux, the long-dead Lunar Elders remaining as the last of the old-guard in recluse, holding sizable but unworkable powerbases in the Underworld and falling back against the threat of unforeseen population growth in destitute ghosts and the self-styled Deathlords and their Abyssal Exalted taking advantage of that sea-change. The old barrier infrastructure has broken down but the fringes of the wyld still hold sanctuary for the Lunars, as it helps mask their activities in ways the Underworld and its shadowlands had previously done. With the Scarlet Empire weakened, if there was a time to act, that time is now.
Because like the Solar Exalted, they are Alive, they are Willing, and they have Returned.