Alright fine, I'll give you this in that you're not trying to argue that Disney executives are some controlling corprate force that will or wont allow Lucasfilm to make what they want because money, we've had that discussion numerous times today with people using the terms comittee and story comittee. Which we've already established isn't true.
So going from there, from what I've read about production and heard from panels, interviews etc.
Each film maker gets to tell the story that they want to. This isn't up to anyone else but them. The Force Awakens was made by JJ Abrams and Kasdan. Starkiller base resembles the death star because that's what they wrote and wanted. No one stopped them from taking bigger risks in the star wars universe, that was entirely their choice, and while they could have had something better than Death Star 3.0, I think starting off the trilogy that way is a good way to get people back into and interested in Star Wars again, so it served its purpose, it got people interested, it introduced new characters, and most people like it.
Moving on from there, while Kasdan and JJ Abrams might have some input and suggestions on where the next film goes, its not up to them. JJ right now is only involved in episode 8 as a producer, he has little to no creative part of Episode 8, neither does Kasdan as he's working on the Han Solo film. So just about everything in Episode 8 is the way Rian Johnson wants it, as he's both the sole writer and director of the film. That's not to say he has absolute control over every single aspect, but lets say its about 85%-90%.
There isn't any set direction for this trilogy to follow, and the films content aren't dictated by extended universe content, its the other way around. The films will dictate what happens in the extended universe content, so the films have more room to breathe and extended universe is there to compliment the films and the universe rather than congest it, which is why the old EU was thrown out. There was no where to go and no room to breathe. The lucasfilm story group works with the directors on how the Star Wars universe goes forward, the directors have far more say than the story group, but it's still a collaberative effort and again, the only authority that the lucasfilm story group has, is to say "You can't do that because it would contradict currently established canon" or "Okay so, you want to do this, we'll give you room in the extended universe to make sure no one is working on something similar so there's no overlap or contradiction". That's pretty much it.
From there, of course Kathleen Kennedy will oversee all projects, she decides what gets greenlit, she decides who she wants directing the films and of course she's watching over to make sure everything fits together and goes smoothy. She's pretty hands off, and if I recall correctly, she specifically called Episode 8 "Rians vision" or something along those lines. That means that episode 8, its direction and its content for the most part is entirely up to Rian, in the same way that Episode 7's content was entirely up to JJ and Kasdan and its pretty clear that Kathleen herself is entirely okay with the hands off approach, that in particular was something I think was brought up during TLJ panel. But the most important part of this is that this film will be his vision and its up to him to create the future of Star Wars and its direction.
Taking that into account, also know that Rian wrote the story for Episode 9. Which means that this trilogy is pretty much Rian's trilogy and that he has near complete control over Star Wars' future. If they didn't trust him to do that job then he wouldn't have it at all. But unless Rian wants to start putting rape scenes and things like disembowelment into Star Wars then he's got the keys to do whatever he wants. Colin Trevarrow doesn't have to follow what Rian wants and he's free to change what he likes, he has the final say on episode 9 because it'll be his film and if he wants to do that, no one is going to stop him as long as it fits with everything else.
Now on whether or not Rian will actually do that: It's been stated that Episode 8 is a very different star wars film and that Rian is doing a lot of new things. If they didn't trust him and if he wasn't able to work freely you wouldn't be hearing these things at all. But if anything these films are more director friendly than things like Marvel films and its entirely up to Rian how far he wants to take his ideas and concepts, and the future of this franchise rests in his hands.
there isn't any preset story and no one is going to step in unless they have to.
