No, i dont think that your speculation is right, not about the reasons behind the multipart format and not about this being only 2 parts.
The first objection is because imo what you claim doesnt make much sense, 'cause if THAT was really their original intention from the beginning (and it must have been since they said immediately that this was a multi-game format, it's not something that they said on a second time) it would have been 1000 times more logical to do this whole project inside nextgen instead to divide it like this that sure as hell can only bring extra complications, not to mention that they will probably do a ps5-version anyway eventually (unless the ps4 version already includes options for nextgen that will become available when playing on ps5....but i doubt it since it will probably be better for SE to release a separate ps5 version in the future).
The second objection instead is a more practical one, doing only 2 parts would mean having the second part to cover everything else outside Midgar, and unless they really change the narrative flow from part 1 it will become something waaaay too big making that an evidently bad strategical decision for SE.......and if instead SE decides to change the flow from part 1, then it will be impossible to not notice and have the feeling that the whole second part is rushed, or to be more precise, as if the story is played in fast-forward.
Nope, that's not good writing AT ALL, if you start to use a pace in a story you have to go on with that pace or the final result will feel disappointing and im pretty sure that as i know something so basic even at SE they are perfectly aware of this and for this reason i will be extremely surprised if this project wont have at least 3 parts (or more, depending how much they want to expand things in this remake and even if they want to go beyond the events from the original game, who knows....)
"Remember, you can make anything as idiot-proof as you want, they'll just build a better idiot...."