SPOILERS BELOW.

If you Google around searching for information about Maia's companion quest, "The Courier's Calling", you'll see lots of advice that if you don't get rid of the stalker for Tuaha, and just convince her to take the missive, that this will change the ending for Port Maje (i.e. Governor Clario doesn't get assassinated.).  And, in fact, when discussing the mission with Atsura, he will say that Tuaha was forced to leave the island prematurely, which certainly suggests that the assassination attempt failed.  But, in fact, at least in my game, I got the assassination ending anyway and I can tell from that Googling that I'm not alone in this.  From looking at the code, as far as I can tell, the only way to ensure a good ending for Port Maje in vanilla is to never do the Port Maje portion of the quest at all, to simply never talk to Tuaha.  This is needless to say unintuitive and unsatisfying, pretty much requires metagaming knowledge to pull off, and makes the reward for the quest unobtainable unless you're ok with dooming Port Maje to riots and death.

So I dug into the files, figured out what was wrong, and fixed it.

How I did it:  The original endgameslides.conversationbundle file checks on the variable n_Quest_Tuaha_Success = 1, but this is set in Tuaha's dialogue inconsistently between various dialogue branches, and also inconsistent with Atsura's dialogue.  The way Atsura's dialogue checks the quest states seems much more logical and consistent.  So I changed the ending slides to work off the same variable that Atsura's dialogue checks (n_Quest_Tuaha_State = 3), so with this mod the ending slides will always be consistent with what Atsura stated as the results of your actions (Tuaha didn't flee = assassination success, Tuaha fled = assassination failure).

I consider this a bugfix.  I just can't see the way it "works" in vanilla as intended.  If you disagree, you can always simply not install this, but I'd certainly be interested in hearing a good counterargument for why this shouldn't be considered a bug fix and how the way it works in vanilla actually makes sense somehow, given Atsura's dialogue about Tuaha fleeing.  EDIT:  After releasing this mod, I received information from the developers at Obsidian that confirm this is not actually a bug, that whether or not the assassination succeeds if you don't deal with the stalker is a random 50/50 chance.  See the sticky post for details and a link to the conversation with the developer.  I have therefore relabeled this mod as a Tweak rather than a Fix.

Note:  It's possible that some specific dialogue paths with Tuaha will work correctly... it's hard to tell as I'm not seeing a clear connection between the nodes that set the variables and the corresponding dialogue nodes... but for sure at least some, like the ones I naturally picked, don't work.  I'm really not willing to replay 50 hours just to test every possible Tuaha dialogue branch and then get to the ending slides.  But if anyone sees this mod not work as explained and expected, please let me know. 

You should still get the appropriate and intended slides if you kill Governor Clario and/or Ikawha yourself.

Note that if you do get the good ending for Port Maje, this can extend the Tikawara ending by an additional slide as well, if you also prevented that assassination.

Installation:  Extract "PortMajeEndingFix.7z" directly into your override folder.  This should create the following folder structure inside override:  PortMajeEndingFix\Conversations\endgameslides.conversationbundle.

Compatibility:  This will conflict with any other mod that changes the ending game slides, and *cannot* conflict with anything else, but in my searching on Nexus I couldn't find any other mod that touches the end game slides.  Please let me know if I'm incorrect and I'll see if anything can be done to make them compatible.

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I've made a number of other mods for other games that you might be interested in:

My Fallout New Vegas Mods:

QRNVR3 - Qwinn's Refined New Vegas Redesigned 3

My Skyrim Special Edition mods:

QUASIPC - Qwinn's Unified Automated Self Installing Patch Compendium
QRVAE - Qwinn's Refined Visual Animated Enchants
Improved Redbelly Mine Mist Fix  (Replaced patches for ELE-USSEP and Luminosity-USSEP)
QUARK - Qwinn's Ultimate Amulet Restoration Kit
QUSTR - Qwinn's Universal Skin Texture Replacer
Scoped Bows Compatibility Kit
IDRS - Improved Disabled Race Scaling
Critters Ain't Snitches - Animal and Monster Crime Reporting Fix
Interesting NPCs Amalee Customizer (by frogf, port only)
Interesting NPCs Zora Customizer (by frogf, port only)
TMBE Enhanced Body for SSE (by Robton, port only)
TMB Vanilla Armors and Clothes (by Robton, port only)

My Dragon Age Origins mods:

Qwinn's Ultimate Dragon Age Origins Fixpack
Tougher Merchants

My Pillars of Eternity 2: Deadfire mods:

Port Maje Ending Slides Fix