From what I remember Johnson is, in fact, writing the Episode IX screenplay, though I'm sure there's room for another writer to add to it.
When all is said and done I'd really like to see a comprehensive book on the making of this trilogy.
From what I understand Lucas had an outline that was entirely thrown out by Abrams (by the way, Abrams is a very competent writer and I think he was calling the shots more so than Kasdan). I'd imagine Abrams has an outline or at least an idea of where he thinks the trilogy should go but I imagine his priority was to make his film stand on its own (as many first parts do). I don't know if its been confirmed that he's producing VIII and IX, but I'd be surprised if he didn't as he's still a producer on the Mission: Impossible sequels and the new Star Trek. I think he really wants to move on to making his own stuff as soon as possible but he'll probably be around to consult and give feedback to Johnson and Trevorrow.
It actually makes a lot of sense that they'd have Johnson write both VIII and IX, as second parters are in many ways set ups for the conclusion, and conclusions are in many ways pay offs to second parts. They go hand in hand so its good that one person's there to breakdown those stories.
The one thing I wonder is where is Kasdan in all of this? I know he's developing the Han Solo movie but it'd be strange if he didn't at least do some revisions to VIII and IX to keep them consistent with each other and the OT. There's certainly enough time before those films come out for changes. I'd honestly be surprised if Johnson ends up being the only credited screenwriter on both. To me it seems like he's laying down the groundwork for later drafts to be written with Kasdan and (in the case of IX) Trevorrow.
When it comes to the visuals, I'm not all that worried. I hope that Trevorrow tries to remain consistent with what's come before but I do want some visual differentiation between the films which is part of the point of the different directors (though if Trevorrow even thinks about applying a color grade like in JW I'll be pretty angry). I think Trevorrow actually wanted more practicals in JW but didn't have the clout to make it happen. Obviously now he has a mega blockbuster behind him and it's become almost a mission statement on the part of Lucasfilm to go as practical as possible (and there's reason to believe that Johnson will go even further in that regard than Abrams). And I think it's safe to say at this point that the whole trilogy will be on 35mm (save the stray IMAX scene).