I wondered about that, too... my oldest was the height your daughter is when he was 6.5 and he outgrew the Regent AND the turbobooster then.
what I would probably consider doing is, since you are putting the efta in the basement anyway, is investing in a SK radian... put that, the scenera, and the other booster in the third row, and put your 7yr old in a taller highback booster (SK Monterey, Britax Parkway SG, Recaro Probooster) in the 2nd row with the baby.
in fact, with a radian, if the monterey fits in the third row, you MIGHT be able to pull off Monterey, ff radian, rf scenera, and have the younger booster rider up next to the new baby depending on how you want to do it.
However, budget might not allow that or it might not be doable in other ways (since I don't know if the child currently in the nautilus would be able to buckle themselves)... and if not, then I think you've done the best you can and if you need to, a backless booster isn't the worst thing on earth.
I'd recommend the cosco backless or harmony backless boosters (literider or cruz) simply because they are good for 3-across.