Explain an ending that confused you

I've just finished Evangelion, the End of Evangelion film and I'm confused.

What I think happened: Everyone turns into orange liquid and joins together, then Shinji decides he doesn't want that and comes back, and he's on a beach with Asuka and he strangles her. I don't know if any of that is real or in his head.
What lost me: Why he chooses to come back when the whole film says being alone hurts. And what Asuka's last line means.

Assume I have finished it, so spoilers are fine.

Start by telling me what happens, plainly, in under 150 words. Plot only. No themes yet. If I've got something wrong in my summary above, correct it here and say which bit I misread.

Then, separately:
1. What it means, in the reading most people take. Point at what in the film or book supports it, not at what critics said.
2. My specific questions, answered one at a time.
3. The parts that are genuinely unresolved. Not "open to interpretation" as a polite dodge. Tell me where the work deliberately refuses to answer, and where the author has said outside the work that there's no answer.
4. One respectable reading that disagrees with the main one, and what it hangs on.

If you're not sure about something, say you're not sure. I'd rather have an honest gap than a confident answer I later find out is fan theory presented as fact.

What you get: The plot in plain words first, then the meaning, then a straight list of what genuinely has no answer.