I think it would have been sawyer in the coffin because Kate only seems to talk with the girls otherwise and they would have had other survivors at their funerals while sawyer is still an outcast realistically. Maybe something happens between Kate and Sawyer? Oh and yeah, it's definitely a flash forward now that you guys say those things and I was thinking about those physics before the grenade went off
And realistically, even given the density of the water down there, the grenade wouldn't have blown the glass from that distance and with the thick glass it would have to be (after all the guy swam away from the glass a bit). And one last question.... how did Penelope know to send a signal out there right at that very moment the jamming equipment wet down?