You don't need spoiler marks, topic already has spoilers in the title so you're safe. --- I am a dedicated member of the "Walter Sullivan Is Bad-Ass" group!!! I am the true originator of the Cookie Demon theory on the SH2 and 3 boards.
I don't know if they're dead. After the credits, there's a voice that says "Ellie...Ellie" and I believe that it was Isaac. So, perhaps he's still alive. Or he said that as he was falling back to the surface of Tau Volantis. Not sure.