S.D.
I would keep her in the Marathon. Britax seats are wonderful, I have both of my kids in them. If she likes it, keep her in it. (My dd is also 5 and still in hers, and doesnt want to give it up for a booster.) Warm the car up before she gets in, when you can, otherwise have her take off her coat to get in it. Then cover her with a blanket or lay her coat over her. Keep hat and mittens on her, as well. It is safer for a chld to use the car seat with OUT a big winter coat, anyways. They fit in it better, you can adjust it tighter, and they will be safer if you were in an accident. She should be able to stay in it for a long time. Also, pads or straps or anything you buy in the store, out of a catalog, etc, that did NOT come WITH the car seat, should not be used. There is no way of knowing how safe it is and it could change the effectiveness of the car seat.
S.