Hi, I'm paying attention now! I have a VW Golf (very similar to the Jetta's in the back seat from what I understand) and a Mazda 5, and I'm completely obsessed with carseats so I've tried quite a few in those vehicles, so hopefully I can give you some suggestions.
I'll just give you a run-down on the seats I've got/tried in my vehicles, and then go back over the thread and try to find any specific questions I can answer. I'll add pics to show you how about how much space they take up.
The Radian:
Installs beautifully in the center of the Golf - like it was made to go there! Slightly more tricky outboard, but not bad. Leaves a lot of leg-room for the child:
The Radian is incompatible FF in the third row of the Mazda 5, but easy to install in the second row:
The Regent:
Installs fairly well both center and outboard in the Golf; it takes a few tricks and learning how to install it, but after that it's simple. Leaves a couple inches less legroom for the child than the Radian, though:
The Regent installs pretty easily in the second row of the Mazda 5; a bit tricky in the third row but quite possible:
(second row)
(third row)
Unfortunately, I haven't had a chance to try out the GN in either of our vehicles, though I intend to do so as soon as my back is better (could be a couple weeks). However, I really don't anticipate it being a problem in any of our vehicles in any seating position, as the belt path is fairly high.
I think you'd be happy with any of the three in either vehicle, actually - the Regent, the Radian, or the GN. If I were buying two carseats brand-new right now, I'd probably get a Regent for the Mazda5 and a GN for the Golf, though. Much as I like the Radian, unless you'll be doing a lot of traveling by plane or squeezing it into 3-across situations, it's a small seat that won't fit your child as long. My dd in the pics above is very petite - age 7 but just 40lbs. You can see how the Radian fits her very well, but an average 7 year-old would be starting to get squished. Plus, I don't like the fact that it's incompatible with a seating position in my car - I like having options, and with the Radian, I simply don't have any choice but to put it in the second row.
I notice you asked about putting something under the carseat to protect the leather vehicle seats - a thin towel is acceptable. The seat protectors they sell are too thick, and they aren't safe, so don't waste your money.
Hope that helps! Feel free to ask me any other questions you may have. I love both our cars and playing with carseats is my hobby. :thumbsup: