Katie, why am I not surprised you were in a town's citizens' police academy?? :D

I was in our town's citizens' police academy a few years ago. Here if you're in the back seat, you're not getting out until they open it for you. (If there are handles, they're not operational, but I don't remember if they're there.)

At our station, if they have someone in the back they pull into a big garage attached to the station and lower the door. If the person makes a break for it when they get out, there's simply nowhere to go--they can't get out and the door into the station is opened from the inside.

The cars tend to get thrown up in a lot, so there's that. And a thing I hadn't known--the front console has a bunch of different siren sounds! Our police cars have fifteen. (You bet I hit those switches, and it was very fun.) They tend to vary their siren sound as they near an intersection because it gets the attention of drivers who may not be paying attention. (The officer I rode with says they don't typically use all the sounds because it would just confuse people--they have two or three go-to sirens.)

I don't know the answers to your exact questions, but I have sat in the back of a police car one time. A few years ago on an extremely cold day in North Dakota, I got t-boned by a student who was driving a little fast for conditions and could not stop. It was just BITTERLY cold and my car had a huge hole in the side of it. The police officer that came to the accident had us sit in his back seat so that we could exchange insurance information and call our agents (we couldn't feel our fingers otherwise). What I remember through the shock: the seat was very hard vinyl, not comfy, but easy to swab off if I guess you were bloody or dirty. A slippery seat. There are bars between the backseat and front so that an arrested person can't reach up and attack the driver. And there was NO leg room. I am 5'2" and my knees were basically pressed against the driver's seat or barrier or whatever was there. Just very claustrophobic feeling; I was glad to get out of there.  I honestly don't remember if we closed the doors all the way (probably), or if the officer opened the doors for us. I don't remember anything extraordinarily different, but I would assume that if they are picking up a murderer, for example, they probably have the child safety locks on so the people in the back can't open the doors and run away.

You might be able to find other video tours of police cars, but here's one. https://www.youtube.com/watch?v=q2c-AHwjJE0