We used to drive an 04 ION and I know 3 ff'ing would definitely work as long as kiddo's were under the LATCH weight limit. I had made only a hasty attempt at a seatbelt install when trying the seat in the car and hadn't been able to make it work, but also had no experience with the seat at the time.
Getting it to fit rf'ing would definitely be tricky. I know that we were able to get the AO to fit at a newborn recline in the center as it went between the front seats. I'm not sure how much further forward the top of the radian would go though - it might interfere with the front passenger space.
I was able to fit a rf'ing MA in behind the passenger seat at an upright angle for a toddler, here's a picture of what it looked like - note that my dh is 6' tall, so the seat is adjusted for him. He didn't have much room, but it was just enough. (my dd wasn't being the most cooperative model.
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In that picture there is also a snugride installed center. It was a very tight fit - I had the MA positioned right against the side of the seat so that the door handle was basically right against the seat when the door was shut. The Snugride did angle very slightly when the seat was snapped on to the base. The MA and snugride really don't puzzle well together at all though...
So I'd say that Radian's outboard you'd probably even be able to get a carrier in the middle. I'd try something like a SS1 that would let you get the baby past a year old and to a point where he/she would be able to handle a more upright angle.
Knowing what I know now, I think I probably could get the radian installed with the seatbelt. I think it would need a locking clip in order to keep the belt locked though. IIRC the seatbelt was tightened enough trying to install the seat that it kept on switching back to the emergency locking mode instead of staying in locking mode. Unfortunately I don't have the car anymore or I could try the configuration out for you...
But the ION does have 3 LATCH positions, so I think with the radians outboard you'd have some good space to work with in the middle and shouldn't run in to any major incompatibility issues.