3 across in a 99 Civic

I have a Marathon and a (4.5 year old) Roundabout and plan to get at least one Radian in order to fit 3 kids in my 99 Civic. I went to a store and tried MA (FF), RA (RF), Rad (FF) and the arm rest of the car door kept the Radian side's door from closing by maybe 4 inches. But it was dark and I was in a rush, so I didn't try every possible combination. They also only had the one Radian to try. I have long-torsoed kids -- 4.5 year old, 43", 39 lb; 2.5 year old 37", 31 lbs, and I'll be nannying a 16 month old. My two year old is forward facing because he out-heighted the Roundabout rear-facing. After checking out the 3-across thread, it seems like 2 Radians would probably work with one of my Britaxes -- hopefully the Marathon because the Roundabout's closer to expiration. We're planning on having a third baby in the not-too-distant future, so I don't mind spending for better seats and the nannied baby's mom might help me buy a seat for her child.

I'm trying to figure out if I should give up hope on being able to purchase only one seat. Has anyone done the MA, RA, Rad combo in a Civic this old? Radians are on sale at the store this weekend, but they don't have two to test (I can order them at the sale price).

What is the store's return policy? If they will refund 100% of your money, you don't really have anything to lose by ordering two.