Also a fair point, but I still feel that the back half of the cinematic feels a bit dragged out and could use to either be shorter or have more action. I mean no offence, but I had the impression that this part was more you showing off your terraining/cinematic-making skills, than something that would be actually relevant to the story.
Another fair point. It's good to see that you seem to have a perfectly logical reasoning for every little bit of the cinematic
That said, if it's a vision... well, visions were usually something that was granted for a pretty specific purpose, regardless if that purpose was clear initially or not. In your vision, I don't really see a meaningful purpose - it appears to just inform Sen'jin that there was a naval battle nearby and that humans and orcs have landed on his island.
That's a little ambiguous, which might be a good thing if that's what you're going for, but I think it could use to be a bit more structured, i.e. be more of a warning. Thus, I feel it might be a good idea to show that the humans are attacking the trolls inside the vision. Or maybe, if you don't want to be so direct, do something like a vision of a troll village with human forces coming from all sides. Something that would more or less directly portray humans as a threat to the trolls.
Finally, I think you could consider moving the Thrall's part to the end of the vision sequence. In my opinion it would add a little more structure to the vision, i.e. it would first show the context (sea battle), then the threat (humans attacking the trolls) and then the hope (Thrall and the Horde) - it could make the vision feel more like it's directed to warn the trolls and push them towards seeking out the Horde, which is what they do in the next mission.