One method you could do is the alternate realities. The events that happen in Hell Rebels are not the same events that happened for Age of Ashes when they arrive there. It would avoid situation of Barzillai still being alive even if the party in Hell's Rebels fail to kill him.
But if you had to run it as IF Barzillai Thrune is still alive and ruling over Kintargo, then a few things to be note,
- The Remnant of Barzillai and his haunts should be replaced with Remnant of <Character's name from Hell's Rebels> or some other threat that the Raja Rakshasas use to cause a disturbance in the bar.
- Sunset Imports is thriving with good business dealings in Cheliax and so far beyond with the House Thrune contracts in place. Thus the Scarlet Triad don't have intimidation that a failing business owner would have. Instead the relation is now a leveled business arrangement with neither side having an advantage over the other. Sedranni allows the Scarlet Triad to bring slaves back and forth from Kintargo without a qualm or issue long as she gets paid for the fees for storage or usage of the docks to load and unload.
-The Bellflower network, although secretive, would need to work even more in secrecy and less obscure methods to avoid Barzillai's guards from finding them. When they find Hundy among other soon-to-be sold off slave being tortured. His perseverance should be a bit stronger knowing that Scarlet Triad could relay this knowledge to Barzillai to remove a rebellion from his city. The party would need to earn Hundy's trust before leading them to the safehouses.
-The ambassador from Nidal would be another path of gaining Barzillai's trust as the Scarlet Triad enslaving one of Nidal's ambassadors would be distasteful act for diplomacy between the nations that Barzillai would listen to their case. He decides to name the players as auditors to review the Scarlet Triad's business and wants a throughout investigation of their work if they've been enslaving other highly-valued citizens that aren't on the list of being enslaved. The players would be given leads such as the Tanessan Tower and Sunset Imports. The Scarlet Triad seeing these 'auditors' decide to take care of them by some measure before they find a group of slaves that are part of Barzillai's higher court being enslaved and another portion of high court in the Summershade Granite Quarry to work on finding a gem that the Scarlet Triad is interested in finding. Annoyed, Barzillai tasks the adventurers on going to the quarry to free the slaves that are part of his high court.
-Book 3 if ran with the events that Barzillai is alive shouldn't be a means of fulfilling the end goals of another party, instead it should be living in the consequences of what that party did or didn't accomplish. They have to work covertly with the Bellflower Network in doing what rebellion that they can or working with a evil ruler that doesn't care about the plight of his citizens being enslaved, only with his trusted higher court being taken from him.
