I'll drop my 2cents here.
I don't think the coffin represents a multiuniverse because they are visible and tangible simultaneously on a particular timeline. Also, if they were meant to represent multiple universes, there wouldn't be one coffin opening at a time, all would be closed or opened because it would happen
simultaneously. Instead, the lined up indicates a sequence (a circular one with Shin Eva) of a cyclic story that repeats and repeats.
Also, in Shin Eva when Kaworu told him they met many times in the past. Shinji replies "Yeah, I remember now. We've met many times
here" while indicating the lakeshore with the headless statue (NGE). So Shinji is the same Shinji, since Kaworu specifically met him due to this Book of Vagueness after their first meeting.
As what this means about being a sequel or not. Who knows. I would rather not, in a strict sense (I'm fine to think NTE as a spiritual sequel), but I can't say for sure. I hope Anno will clarify things about this in next CRC. Aside from the things mentioned above and all the vague winks and implications in previous movies, is that Kaworu calls Instrumentality the "origin of the Cycle."