The armrests on the bulkhead seats don't move, but the others do; if you lift the armrest the seat should go in fine. The story of the buckle is making me laugh; I wouldn't worry about that happening, and if it does, either go to the seat behind and reach up between the cushions to unbuckle it, or remove the upholstery from the carseat to reach the buckle. The location of the buckle almost always does cause some problems, because in cars the buckle is side-mounted, but in aircraft it is supposed to hit right in the middle. It's usually easiest for Mom to secure the seat in an aircraft; smaller hands.
What kind of seat do you have that it is that wide? Is it a bar-front? If so, don't count the bar in the measurement; it is the width of the base and seat bottom that counts. Our convertible seats were Evenflo Ultara V's; I can tell you from experience that an Ultara is a bad seat for travel, not because it is wide (though it is), but because it is really heavy to carry. If you do have a bar-front seat, you will probably find it somewhat difficult to secure, because it must go in the window seat, and it's hard to reach around the bar in that cramped space.
As for the narrow seat for travel, the all-time champ in that category is the Cosco Touriva 5-pt. convertible. We had one for our "grandma's house" seat. Wally-World and K-Mart usually have them for less than $40. Not a super-cushy seat, but it worked fine in every rental vehicle we ever had to put it into.
We currently travel with a Cosco E.B. 5 pt. booster; this is the first time we'll be checking it, as DS just broke 40 lbs. We have a Prince Lionheart backpack carseat satchel, which makes schlepping it through the terminal MUCH easier.
