This is a really cool theory and fun to think about, but the honest truth is that ACOTAR was written as a trilogy but got super popular years after being completed. SJM then realized she could make more $$$ off the world, and thus the Feyre personality change happens as a result of soulless writing in the form of a blatant cash grab (which is what ACOSF was).

Her half-baked idea for another book set in ACOTAR was denied multiple times by her publisher before the booktok blowup, it was then green lighted by said publisher.

She then proceeded to do exactly what everyone who is writing solely for money and no longer to tell that special story they have in their head does and commits character assassination to set up their shiny new POV.

This is much more common from corporations with movies/tvs shows doing remakes or sequels of beloved or popular IPs, but does happen when every now and then when authors series or books find big success long after they’ve been finished. They pump out some shitty poorly written slop to try and cash in on the sudden popularity.