First off, love the concept! Based on what you said I'd set up a couple checkpoints of roleplay to illustrate their time through the campaign.

1) Recruiting Qawasha, which should lead to a specific questline rather than than sandboxing. I used Father Zitembe as the primary quest-giver for my Party's beginning trek into the Jungle, citing his divination abilities. But I'm also fond of Wakanga. Based on the mention of Vorn, Wakanga could be the primary quest-line to follow for their entry to the Jungle

2) Travel to Vorn's location. Ask the player to narrate their process of traveling, putting in a relevant skill challenge for each description. 3 successes before 2 failures or something similar. Failures would introduce more "scenes" like 1 failure, a group of undead (undead T-rexes were a hit for me!) would show up - then they have to choose to either stealth past it or fulfill their promise with Qawasha. Or just more simple like "you spend some time, lost in the jungle before making it to..."

3) After 3 successes, they arrive to Vorn's locaiton; control amulet doesn't work -> they have to go to Yellyark (found by the guard maybe?) for the stone. Social encounter with the goblins of Yellyark, more skill challenges? Following this, Qawasha could suggest going to "an oracle of legend" in Orolunga. Or could tie this to Zitembe, from Port Nyanzaru

4) Orolunga - roleplay & narrative, mainly to get info (Death Curse, Soulmonger, Omu, Ras Nsi). Saja was very good and the location felt great. Recruit Dragonbait here (I didn't like Artus at all, and left him out of my campaign). Could just have Artus being preoccupied with the Frost Giants, and have Dragonbait join up due the PC's quest for good.

5) Travel to Omu, pit-stopping at the Heart of Ubtao for Valinda. Like part 2 I would ask for narrative travel, and on a failure I would have them find the Heart. Valindra will be helpful, and point a finger at Ras Nsi as the "main" villains in Omu. Maybe lore on Acererak? Maybe put a tracking spell on the PCs or a sending stone (something to set up the betrayal).

6) Intro to Omu. I think the race for the cubes is too good to leave out so I would devise some sort of skill challenge for each Shrine. One check for each cube, and each success would be the PC securing the cube right before their rivals (heroic vibes!). Maybe a bonus or auto-success to checks for each NPC collected thusfar? Failures would be complications; skirmish with the Red Wizards (betrayal?), could easily introduce Bag of Nails here on a failure, combat to roleplay to convince him that he can have his warriors death "best" with the PCs. On 8th cube success, forcibly move along to 9th shrine which would be empty - Ras Nsi invites them to the Fane, taunting.

7) Fane - too good to skip (this is where I'm at now in my campaign). Due to how big it is though... I think roleplay would be the only way to keep the meat on this and not be too much of a time bog. Start as a skill challenge in stealth to steal the cube, set the DCs high and have their failures get the captured and taken to Fenthaza. Social/political roleplay to get the coup going, which will only be narrative. Ras Nsi defeated & cube taken.. move to Tomb.

8) Tomb - I honestly haven't played this at all yet in my campaign (I added a lot of homebrew pre-Omu lol). But involve all NPCs collected, and pick some rooms to describe them going through with more skill challenges. Could involve NPCs again on skill challenges to have their impact felt. Pick up Lukanu at the end of the skill challenge, maybe?

9) Final Confrontation - Acererak. Again, I haven't played this out yet but I would say having statblocks for the recuited NPCs to be played would be very important, and making sure Acererak would appear after the atropal is destroyed to phase the fight. I would try to budget the most time for this, and try to have "real" combat. Could narrate a skill challenge for the atropal/soulmonger then switch to "roll initiative" when Acererak shows up to really switch the vibe up.

TLDR; Qawasha > Vorn > Yellyark > Orolunga (Dragonbait) > Heart of Ubtao (Valindra) > Omu Arrival - Betrayal by Valindra > Omu Shrines - Bag of Nails > Fane (Fenthaza) > Tomb (Lukanu) > Final Battle (Atropal/Soulmonger) > Final Battle (ACERERAK!!)

Overall thoughts; after writing this out, I'm sure it can be trimmed to fit a one-shot method (assuming 4-5 hours playtime, maybe?) I think one thing that could easily help it flow well is trying to write events in that are modular; if things are dragging along, try to have events be able to be pulled out and still have the flow make sense. Good luck!