When did they die?

I totally agree with you! that's my theory too. Except for Juliet's part she died in LAX where they showed us!
And if it was the segundo opinion, they wouldn't have been therough the flashsideways coze they were already dead on the island so that wouldn't be the most important phase of their lives like Christian said, not their inbetween or afterlife

They all died at some point; as does everyone. That's all that matters. Some of them died on the island, some of them died a long time before that, some of them died a long time later than that. The purgatory world is independent of time: they have all died and are moving on together. Everything on the island was real.

Exactly, I agree with your first opinion. That the island was real, they were alive there, they each died there or off the island. The flash sideways in my opinion was "Limbo", before going to the afterlife to find each other, get past their issues and not "die alone". :)

I think it's completely unclear who is dead at the end versus who is still alive. After all, Christian cast much doubt on the life or death status of everyone in the church when talking to Jack. I believe that Christian was debunking time itself. "There is no now here." The place they made so the could all find each other was just that, a place. Not a time. Yet, there was mention of "when you're ready" to Jack and "she's not ready yet" in reference to Ana Lucia. I don't think any of this portends to answer the tanong "when did they die?" On the contrary, I think it attempts to convince us that the issue of when they died doesn't matter. Was any of this effective? I vote no. The vague approach to it all haphazardly dishonored the rest of the ipakita in a way that is forgiveable but disappointing. Maybe I'm just not ready yet. And whatever happened happened.