Finality?

The alternate reality, where they were all back in LA and Desmond tried to get them all back together, that was purgatory. The only way they could move on was by getting together to realise their time on the island with each other was the most important part of their lives. So they remembered this, found peace, and could move on. Ana Lucia and Ben couldn't move on yet as they weren't ready, they're waiting for their own peace.

Everything that happened on the island actually happened. Jack died there, lying next to Vincent, like when he arrived. Hugo went on to protect the island, eventually dying (how, we don't know) with Ben as his number 2 (hence the comment by Hugo in purgatory).

The people who got on the plane all took off, got back to America, and lived full lives until death.

It was in death where everybody was waiting for Jack to "let go". Once he did, everything was resolved and they could move on.