I don't think the Incident destroyed the island. They all are on it still at the Temple. The island at the bottom of the ocean I would think is because of something else, like a turn of the wheel or it's hiding. Islands just don't sink when a bomb goes off.

A lot of Dharma was evacuated prior to the bomb. Probably just those left are gone.

Penny could still be alive. She was born off island. Widemore took many trips to the mainland. It is what got him banished. She probably was born prior to 1977. Does not mean that her and Desmond are married of course.

We don't know if the Others would have died either do we? Richard said he watched them all die. They could have been protected somehow.

All we know is that the bomb went off. The hatch was still built in one timeline, possible/probable in the alternate one it was not.

Truth is we really can't figure out stuff yet because we don't know all the connections still that could cause a ripple effect to change people's lives. How is Christian connected? That would play a big role. What about Hurley's father, Micheal's ex-wife, Nadia, Kate's father etc., these are characters that have been introduced with missing "time" that could have connected them to the island prior the incident.