I guess I am wondering if the power-assist is taking more priority than the chair. If she is going to be using a chair all day, the chair should be the first focus, and the power-assist should be secondary to that (just my 2 cents). Yes, they need to work together, but if she is uncomfortable in the chair, no amount of power-assist is going to fix that.
I knew nothing about Ullrich congenital muscular dystrophy (UCMD) but I just read a bit. One thing I saw mentioned was proper sitting posture to prevent curvature of the spine (scoliosis). There are quite a few on the forums with experience with this, and a good supportive backrest is probably really important if this is a risk for her. Have the OT's/PT's given any other advice, other than the joystick? I really think they should be and you should push for it as much as possible.
Have you also looked into a chair like Icon? Gordon can tell you more about it, but I hear it is good for a growing kid with a lot of adjustability and many options for specific needs like supportive backrests like an ADI. Another to look at is the TiLite Twist.
Other than the rep not being so responsive for SmartDrive, I still think you might want to demo it if her upper body control is good. It has many of the requirements you have mentioned, and won't add a lot of weight or width to the chair.
I hope I did not overstep with these concerns/idea's.