I thought it was pretty straight forward.
The Island and everything on it actually happened.
When the last of them died in real life they came back to Flight 815 to find each other.
There is no count of years between the end of the series and limbo, it's not really necessary.
The credit scene with the wreckage was not necessarily from the plane that Lapidus was flying off the island, it could also have been a symbol for everything beginning again. Circle of life, if you will.
Remember, everybody dies, but not always at the same time. There is no reason to think that everyone had to die on the island, or before Jack died. The conversation between Hurley and Ben was proof of that.
As for Ben, I think the reason he chose not to go is that Rousseau and Alex are still there and he wants to spend time with them before they move on together.
And the others that we see that don't move on just, aren't ready.
My biggest, and really only, disappointment was that
Mr.Eko wasn't there. He was one of my favorite characters, and he should have at least been in Limbo somewhere.
But yeah, that's my take on it.