Kael once said that most soul damned for hell go straight to Mammon's hell, but later changed that to Mulcarn's. I prefer to take the compromise position that most start in Mulcarn's hell but few of those deserve hell at all, and that if the evil gods were not cheating Mammon's hell would have by far the most arrivals. The reconquest of the Fane of the Lessers made a serious dent into their ability to kidnap souls, so after the Lord of the Balors scenario Mammon's hell may again be the most crowded.
Ceridwen most certainly does have worshipers: The Emrys. This faith is devoted to destroying the world. It is closely allied with the Ashen Veil (perhaps even seen as an elite order within that faithl) but is probably both far older and more evil. Although it may have changed forms somewhat since then, it existed as far back as Kylorin's reign in Patria. He may have founded the faith or might have joined one already in place and led by Os-Gabella. He probably made it the official religion of the empire at some point, but its worshipers were always few. It is attractive to the elite, not the common man, as it would view unequal power relations as ideal. Instead of providing miracles for her followers, Ceridwen taught them how best to use arcane magic.
The most evil souls, including the Emrys, end up in Aeron's vault. Well, technically everyone starts in Ceridwen's vault but doesn't stay there long, as her vault is the passage that takes them to the other hells.
Given that Emrys means immortals, Ceridwen might show her followers ways to stay alive indefinitely, so they can avoid hell altogether but are still transformed into beings as dark as demons. The Eaters of Dreams may well be priests of the Emrys, and they devour the souls of other mortals in order to gain power and immortality (in a variation, probably the original form, of the ritual used by Vampires) but seem to loose something of their humanity in the process.
Esus's hell is a trap to stop people escaping hell. You normally only get there if you try to repent while in a lower level of hell. Those who end up there may deserve to be in hell even less than those in Mulcarn's hell, as even though they were wicked in life they were on their way to developing the virtues to end up merely in the netherworld or even in a heaven. (I suspect Sirona takes a lot of those who escape hell, and that most seek to escape only meeting angels she sent into hell to show them that it is possible.) The point is to make the repentant disillusioned with repentance so they will backslide again.
There are not many demons in Esus's hell. He prefer living creatures to the souls of the deceased, and the laws of physics in his world are the closest approximation of those of Erebus (although there are subtle differences). I would not be surprised if Esus likes to claim the souls of those loyal to him before they die. The most devout followers of Esus may wander into his world without dying, perhaps never learning that they had left Erebus. They could live on there forever, with all the needs of a physical body. Perhaps they slowly change into changelings, which are the main inhabitants of this hell and which are as alive as any animal on Erebus.
Many followers of Esus though are also very greedy, so they probably head to Mammon's hell and proceed like usual. I would not be surprised if Esus sent his servants into all the hells disguised as Angels of Sirona and with a similar mission to show these souls a way out. Many may work together with real Angels of Sirona and even convince them that they were sent by the Goddess of Mercy with instructions. This is largely just to mess with Sirona and with his fellow evil gods (whom Esus has no qualms about opposing), but would also serve to make sure that they never get a chance to learn how to escape the final trap of hell.
