"The vibe I'm getting is that The Six claimed everyone else was dead in order to protect them. I'm thinking that at some point between the split between Locke's Jack's followers, everyone realized that Naomi's crew (Jeremy Davies, score!) was bad news. They couldn't all hide out in the jungle. Someone had to be found on the island. For whatever reason, these six people agreed to be rescued and cover for the rest."

This is a REALLY good theory and would explain why they want to go back. But it doesn't explain why Kate doesn't.

"What coffin? Oh yeah, is Michael back on the show at some point or not? And Walt? I haven't been following behind the scenes news."

The coffin in the season finale that Jack visited who died alone. It was most likely one of the Oceanic Six. Micheal's back later in the season. No word on Walt.