OK.

I think I learned everything from the other thread that I needed to learn for this little project. To summarize the history of the Lords of the Nine (rather than the history of Baator, per se):
Asmodeus is probably the oldest devil in existence, probably the first of his kind. When he first came to Baator, he brought a number of his servants with him to set up a ruling hierarchy that would serve his purposes. Among the original eight lords he chose to rule his Hell were apparently long-time compatriots Dispater, Mephistopheles, and one who became lord of the seventh layer, whose name is lost to pre-history. In-fighting, backstabbing, betrayals, and assassinations led to the other 5 lords being dethroned or destroyed, and their replacements were in turn replaced over and over by now-forgotten lords in the ensuing millennia. One by one, over time, Zariel, Mammon, Belial, Levistus, and Beherit took charge over the remaining five layers and the Lords of the Nine stabilized and cemented their rule.
Asmodeus was not entirely happy with this arrangement, however, and always sought ways to remake the Lords of the Nine more to his liking. He brokered an unprecedented deal with Tiamat, giving her free reign over Avernus as she saw fit, so long as she helped to keep out invaders and made no trouble for the Lord of the First or the devils therein (Zariel also used this arrangement to make it seem that Tiamat was in charge of Avernus, letting her work behind the scenes); thusly, Asmodeus exerts direct control over “both ends” of the Hells. His perfect prodigy came to the Hells in the form of a fallen archon named Triel. Asmodeus groomed his new discovery, renaming him Baalzebul, and transforming him into a unique baatezu; Asmodeus then removed his old compatriot, the lord of the Seventh, and set Baalzebul in his place. Levistus betrayed Asmodeus by slaying his consort, so he locked the treacherous lord away in a block of ice, and replaced him with the ever-loyal Geryon. Beherit and his consort also betrayed him by attempting to circumvent the natural order of Baator by producing a descendant to replace Asmodeus, so Asmodeus slew them all. Asmodeus offered his prize pupil the opportunity to rule the Sixth layer, so Baalzebul placed his vassal Moloch as lord of the Sixth. With this setup complete, Asmodeus enjoyed nearly full control over Baator for millennia until betrayal came. (Thus, we have the 1st edition hierarchy.)
Baalzebul and his allies, Moloch, Zariel, and Belial, struck against the other four lords, Mephistopheles, Dispater, Mammon, and Geryon (who also wanted to unseat and replace Asmodeus on their own terms), beginning a conflict that became known as the Reckoning. Only Geryon remained loyal to Asmodeus, betraying the rest at a crucial moment. As a punishment to his former favorite, Baalzebul, Asmodeus transformed him into a slug-like creature and banished his vassal Moloch to Avernus; Moloch’s consort Malagarde the Hag Countess was allowed to rule in his stead for a time. Geryon was also removed from his position, with Levistus reinstated in his place. Except for Dispater, the lords of the Nine took on deceptions for a time to change the face of the Hells after the failure of the Reckoning. Belial began to share power with his daughter Fierna and stayed in the background.
Bel, a powerful pit fiend on Avernus began to share power with Zariel. In time, he betrayed her fully, trapping her within the plane and leeching energy from her. Asmodeus was content with the status quo of the current lords of the Nine, and they ruled for many years until the present day. Asmodeus always wished to have his daughter Glasya as one of the nine lords, but her disobedient behavior made her unlikely as an ally. (Thus, we have the Planescape Lords of the Nine.)
In time, Geryon’s despair over losing his position continued to increase, and he was torn apart by Asmodeus. His death released great energies which Asmodeus used to empower Glasya, who had eventually come around to his way of thinking; when combined with the recent death of the Hag Countess, this allowed him to finally install Glasya as the new lord of the Sixth. The remainders of Geryon’s energies continue to linger on as a vestige, a phenomenon that Asmodeus keenly wishes to study for his own use. Levistus and Belial worry that they will be the next to be replaced, and all the other lords shudder in fear at Asmodeus’ displays of power and guile.
With these key points of this history in mind, I’m going to attempt to figure out where the rest of the nobility of Hell fits into the power structure. Who did they serve, and when, and what were their positions, etc.