JJ likely rewrote everything rushed last minute under orders from Bob Iger/Kathleen Kennedy. When they fired Colin Trevorrow, it fell on JJ to pull together all his scribbled unused ideas from TFA. This is why ROS doesn’t really feel like a story and just a string of random ideas–that’s exactly what it was. It almost seems like JJ was told NOT to use anything from the Colin script.
But he actually did use a lot of stuff from the Trevorrow script. A nebula leading to a secret planet, Rey’s parents being killed by a Sith, Kylo sacrificing himself to heal Rey, and Lando showing up with a huge fleet to save the day were all ideas taken from Trevorrow. So that line of thinking is clearly incorrect.
A nebula with a secret planet is hardly limited to Trevorrow. Star Trek: Beyond had used it 2 years earlier, and JJ DEFINITELY knew about that. No one remembers Ochi as a Sith (or a “loyalist”) because he was an unmemorable alien. Trevorrow actually had Kylo do it, which is a HUGELY different plot point than a random forgettable alien; they’re not remotely comparable. That’s like saying Kylo killing Han was the same idea as Starkiller killing Han a few years before.
Sure, you could argue Kylo sacrificing himself was a borrowed idea, but, really, how else would the story have ended under Disney? Other than like 3 ideas, the scripts are lightyears apart. No Coruscant, no Eclipse, no Star Destroyer theft, no Hux suicide, no mutilated Kylo, no Rey/Poe kissing, no stormtrooper rebellion, no double Jedi blade for Rey, no Chewie vs Knights of Ren, no Sith training for Kylo, no Tor Valum at all, no blind Rey, no new Jedi school at the end, no Luke training Rey, no Luke confronting Kylo, no Darksaber, etcetera…so for the 2-3 ideas JJ used, there’s 27 others that were not.