fotoguzzi-
My understanding is that the only seats ejected were those of the rear-most flight attendants. The passengers who were thrown out the back were not found in their seats - whether that means their belts weren't tight or whether the seats failed will be something I assume we will eventually learn.
The question remains about the other girl killed - the one hit by the vehicle. If she was ejected or otherwise exited through the rear, then either she walked or someone carried her to the position where she was struck. I'd suppose it's also possible someone could have carried her down the slide, then laid her down in what they thought was a safe place in order to go help someone else.