I would suggest scheduling an appointment with the carseat checkers in your area. I went to a safety day with the Alachua Co. Safe Kids Coalition.
I did find out something interesting. The inspector told us that the middle is always the safest place, however, the rear-facing seat is the safer seat. So if you have one rear-facing and one front-facing, the front-facing should go in the middle. You could confirm that with them.
That being said, I went almost a year with 2 kids and a Hyundai Accent. That is a TINY 4-door sedan. It would have been physically impossible to put one in the middle and one on the side. The carseats are just too bulky for it to work, so I had to put them on the sides. Yes, it is less safe, but if the seat is installed properly, and you have no other choice you have to go with that.
As for the throwing food and toys. She can't throw something she doesn't have. That sounds harsh, but maybe try making sure she eats before you go somewhere and don't give her toys in the car. I know you need to concentrate on the road and if she's screaming for a toy that's hard, but it's a balancing act you have to get used to. She will get used to it and get over it.