Jack said "we have to get back to my room. It's too high to go out here." Jack/Rebecca's room seemed to open onto a section of roof. I guess Kate's room didn't. However, I agree with everything else you said. He should have told Randall and Rebecca to get out NOW, not "if I'm not back in 3 minutes." Also, when he grabbed the mattress, I thought he was going to make a ladder out of the sheets, not try to use the mattress as a shield. But, quite frankly, if the choices were to go back into a fully engulfed hallway, or jump from the 2nd story window, I'd jump... sheet ladder or not. I wouldn't have even tried to cross the hall. "You and Randall get out now. Kate and I will meet you outside. Somehow."