Yeah, the MR's are the widest seats. When we had 3 and a falcon, we had to put the MR in the garage, and buy a SNS Hi-liner - does same thing as MR - made by same people, attaches to bolt of car, and made for H harness.....its a lot taller and skinner too. We got a SNS compaq rear facing, MR, and the Hi-liner across the back of the falcon....but the MR was defn the prob. If we had of had 2 x hi-liners there would have been even more room...
Maybe save your MR for the spare car ????

In saying all that....MR's are still my 1st choice of any car seat.....ive now got 4 in my car...