This really should have been in R2 spoilers forum - - - however…
This Garcia quest is completed by sheer gameplay, there is no bug, voodoo is not a pre-requisite (as an inquisition supporter)
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1. locate and talk to Angus and he will explain about Corrientes, then leave Angus where you found him.
2. Locate and speak to Corrientes without any aggression then come away, return to the inquisition commander at the dock who will declare Corrientes is Garcia, Scarpetta being his henchman. (if by one of two dialogue choices you take Angus to Garcia he will be shot to silence him)
3. You can also take Bone as a follower back to ship (before talking to Garcia)
4. Make your way back to Angus and tell him to follow you, take him back to port.
5. Now take a suitable follower (eg: Patty) and return to Garcia and confront him with the new dialogue.
6. This part is critical. By game intention, a fight will break but you need to defeat both Garcia and Scarpetta *without leaving the hut* do not exit as natives as well as any Garcia cohorts will attack - - -> you will NOT be attacked by natives etc' if you stay inside the Garcia hut!
7. After looting the bodies of Garcia and Scarpetta, transpose from *within the hut* to anywhere safe (eg: port)
8. As has been mentioned previously in this thread, talk to the remote shamen, then complete 'the blood of the ancestors' quest and return with the result to the shamen, then locate the native tribal leader who at first refuses to believe when explaining Corrientes has fooled the natives. This, and only this last part involving convincing the *native tribal leader* will result in all natives in returning to a non-aggressive state.
Regards
Wulf