Have you considered using a Ride Safer Travel Vest for the car? It will position the seat belt lower. You'd need to verify they height/weight limits for it, but that may be another option. It can pack up small(ish) in a backpack or in luggage.
Another option is coming out at the end of this month. It's called the MiFold. I have one on order. This product also brings the seatbelt down to safely fit on a smaller person/child.
Both of these options wouldn't address the discomfort she experiences in an airplane seat. But airplane seats are just not all that comfortable for anyone. Maybe packing a throw blanket or a small pillow (inflatable, perhaps?) might help make things more comfy.
I'm saying this as someone who rear-faced my oldest child until Age 5.5... A 13-year-old's bones have ossified enough that they are not at the same risk of injury as a younger child's might be in an accident, and so the protection afforded in a full child restraint device is not quite as beneficial or necessary as it is for younger kids. The seat belt does need to be properly positioned, though. And there are lots of options to achieve this without the need for a full CRS.