CyBear,
Congratulations, man are you going to be busy. I have a friend with three kids and I have no idea how he gets anything done.
As far as your seating problem, you're not only going to have to securely attach the seat, but a harness as well, and that might be the problem here. If she's old enough for a three point harness, you're going to have to mount that somewhere back there as well...without taking the interior apart, I'm not sure if you can find a suitable mounting point for the upper support. Even if you can, it'll take some welding to fab that up...
Personally, I don't think the cargo tie downs would be strong enough. You're going to want grade eight fasteners with large backing plates on the other side of the cabin to hold the seat down incase of an impact.
One other thing to think about is emergency ingress/egress. If you have a seat that takes up the entire back, can you get in there and extract her if need be? How about from the second seat? Just call me paranoid. I'd look into a seat that only took up half the back cargo area.
Bogatyr