imo the ending, last half of season 4 is justified. If you take the "Time machine" fan theory and in case of Elliot meeting himself to me it's just happen inside his head like what happen at some moment in S1 and S2.
For split personalities/MPD, if you rewatch the show Sam Esmail definitely laid out everything from start ever since Season 1, that's why rewatching the show especially on S1 some scenes make senses.
Watched this show back in late 2022. Avoided forums/online discussion so I can fully grasp the show on my own thoughts.
SPOILER ALERT
First time I realized that the ending/revelation was that near in S1, jaw dropped really hard.