If your bench seat doesn't have headrests, she is not safe back there unless she is in a highback booster that doesn't require headrests. (I'm assuming you have lap/shoulder belts. If you have just lap belts, only a child in a harnessed seat can ride back there.) How tall is she? She might still fit in a Britax Parkway. If she doesn't, you'll likely need to put one of your other kids back there. How old are the other kids you're transporting, how tall are they, how much do they weigh, and what seats are they in now?
As for the rear crush room, I totally understand your concern. But rear crashes do tend to be much less severe than frontal or side impact crashes in general. And if you have to put a kid back there, you have to put a kid back there. I'm not a tech, so I'll let one of them chime in on which kids might be safest in the rear bench. It will, however, depend on the headrest and belt situation.