G'day
My second
Gehennum campaign was set a century after the first, in the aftermath of the first party's Empire-smashing exploits. The third was set two hundred years before the first, and was involved with the rise of the Theklan hegemony that later became the Empire of Gehennum. The fourth was set two hundred years later than the first campaign, but in the Gehennum that would have developed if the PCs in the first party had never lived (or had failed). Since then I have run lots of campaigns in alternative histories of Gehennum: a campaign in which the evil Samariopolitan League was brought down by the Theklans, a campaign in which it was brought down by Hospis and Borillis, a campaign in which Samariopolis was destroyed by a spellsinger from an obscure little city I've forgotten the name of; campaigns in which PCs make Hyrkanthes so secure on the throne that there is no Civil War, a campaign in which Hyrkanthes defeats Regikhord in the Civil War; several campaigns set in a Gehennum ruled by Regikhord after Jasper defeated Hyrkanthes; a campaign in which Gehennum had broken up in secessionist rebellions after the death of Jasper; several campaigns in which Gehennum had degenerated into a warring clutch of semi-feudal satrapies after the succession of Jokanan on Regikhord's death--in some of these Gehennum is reunited by Darulan the Silent, in others by other episkopoi from Elmis or Asthmara, in others the Dukes of Bethan have re-created the Iron Stone Men and sealed the disintegration of the Empire.
So Gehennum has not so much a history as a set of recurrent historical
possibilities: personalities, movements, conflicts that recur in different versions of its history. This works surprisingly well: players have a good idea what they are fighting for (or against), and they have a rich (but incomplete) set of ideas about such people as Aristarkes II, Hyrkanthes, Jasper, Regikhord, Lesterra, Lysandra, Lykomorphus, Gasparion the Magnificent, Darulan the Silent, Daramalan of Kos, Aspasia.... A player familiar with this mass of stuff can easily play either a Lubber or a Salt, because he or she has played both in campaigns in which Regikhord was a spineless dickhead and campaigns in which Lesterra was a shrew and a slut.
So I say "Go for it!" And don't concern yourself with maintaining a consistent history for the campaign. It isn't necessary and it's often oppressive. If it didn't happen the way you said it did, then no worries, it didn't.
Regards,
Agback