Full Disclosure, it’s heavily based on reading this Reddit post.
I think this forum writer certainly makes the most sense.
The Coherence only comes through observation of the phenomena as per the Schrodinger’s Cat analogy in the film. Meaning, anytime someone was in a black box situation (the items in a box, or Em in the bathroom, or the blackouts) then they switched places in the multiple parallel universes. Em1 traveled through different parallel worlds to one where she had the best, wealthiest life for herself and made all the right decisions such as the Ballet Understudy choice. Also, she did lock-up Em2 (the Universe Em1 entered to steal the life of) in the car perfectly. Pre-Final Blackout, she has the jacket on and post-final blackout, the Em that we see is no longer Em1, but Em3 because she didn’t have the jacket. The Em who comes into the house is from another reality, let’s call her Em4 and she dies in the Bathroom that is said to be unusable. When Em3 wakes-up the next morning after killing Em4, the old lady walks out of the bathroom all wet and with a towel on meaning Em3 is in a reality where it is working. The Em4 vanished into another reality, but there’s still an Em2 in the Trunk because both Em3 and Em4 jumped to different realities in the bathroom by being unobserved by others which is why she pulled the curtain over Em4. Remember, if Em4 is unobservable and switches realities, then so does Em3 because she was also unobserved but had the ring on her. So what we have here is:
Em1: Finds her perfect reality and during the blackout we no longer see her so Em1 SUCCEEDED in getting rid of the other Em and taking her place.
Em4: A version that got out of the trunk and comes crawling in, who is then killed in the bathroom by Em3.
Em3: The one switched for Em1 near the end who no longer had the jacket that Em1 was putting on.
Em2: Who was stuck in the car and called Kevin at the end of the film.
Once the comet passed, this meant:
1. There’s a universe where Emily vanished without a trace.
2. A universe where Emily appeared dead in the bathroom for no explicable reason.
3. A universe where there’s two Emilys’ which is the final scene (which is why #1 exists).
4. The universe of Em1 that successfully exploited the parallel worlds and caused a massive paradox for those three other Emily universes.