Normally, if it is FAA approved, an inch or two over the recommended width isn't going to be an issue as long as you can move at least one of the aircraft armrests. The reality is that if the armrests don't lift, securing the seat is going to be next to impossible in any case.
The difficulty in securing carseats in an aircraft is that carseats are designed to accomodate seatbelts that buckle on the side; aircraft seatbelts buckle in the center of the seat. You need the armrests out of the way so that you can slide your forearm behind the carseat to work the buckles. You can do it more easily if you open up the plane's belt to it's full extension before putting the seat in position and latching the buckle; then pull the free end of the seatbelt strap to tighten it down. The REALLY hard part is getting the seat unlatched in order to remove at the end of the flight -- in some cases the easiest way to reach into that space is to go around into the row behind you and slip your hand between the seat cushions.
The trickiest part of using a wider seat onboard is just getting it onboard. It won't fit down the aisle, so you have to hold it over the seatbacks to carry it on. If you're short like me, the only practical way to carry it and still be able to sort of see where you're going is to hold it upside-down and sideways.