My 3yo twins fit just fine in full size front facing car seats. The seats go in and come out fine. All the latch anchors are easy to get to. Much quicker than putting them in our Durango because the bottom latch anchors are buried deep. The back one is near the floor and the seat doesn't move a lot with a car seat on it. I haven't had any problem with space in the iM. My 9yo even fits fine between the car seats. I'm 5'9" and I ride with the driver seat pushed about halfway back but leaned way back. My car is set up for fun/aggressive driving but I have zero problems hauling my kids around when the need arises. The Durango is the primary kid hauler, though. Leather seats are a must under toddlers.
To compare, when my twins were born we had only a 2007 Hyundai Accent 2 door hatch. We struggled a little getting them in there, but once in, there was still just enough room for the newborn and rear facing seats and a 5yo. The iM rear seat has nearly twice the space and two extra doors. I don't think you'll have a problem.
I love how they gave my Durango all "A's" when it really is a struggle to get the car seats tied down. It's hard enough that my wife gives up and tries to just use the seatbelt slots. And that's a no-no in my book.