I’m not at all ruling out a shot of Vader‘s castle to open us up on Mustafar. However, I think at worst it would be just fine to have the castle show up in the background. I think that’s a good inclusion even on its own.
For the Rogue One establishing shot, I think it’d almost need to lose the lava flow to fit in with how the location is portrayed as healing or growing trees now. If there can be chinks or pieces crumbled off, even better. Now, I don’t know if that’s even possible to do. But if it is, I think that may it really feel like it belongs in the scene. We’d follow three TIEs from space and through a couple establishing shots and onto Kylo.
The castle doesn’t need to be seen with forest around it, but I’d feel better about it without lava.
I’d say It’d be important to implement trees unless we tried to make it appear the forest is behind Vader’s castle and add fog/mist out towards the distant area where the forest would be, could then have the Ties fly into the mist and then go on to the overview shot we already have.