Hi everyone! Thank you all for your suggestions. After HOURS of research and several trips to our nearest (50 miles away)Toys R Us I decided on the Alpha Elite 65. It is a 3in1 car seat. RF 5-40lbs, FF 22-65lbs, Booster 40-100lbs. Height 19"-52" (which is 2" more than typical car seats) 17" highest harness slots (which is awesome!) 3 position recline, 3 crotch slots and the LATCH system (but I used the car seat belt since I was installing it in a 1996 Ford Taurus) It also has support cushions for the head and lateral area. I am very pleased with my decision. It installed easily and isn't overly big. The armrests also flip up out of the way. I will be using this car seat FF with the 5 point harness system in the middle recline position. Hope this helps any other family looking for a carseat for their special needs child. This car seat is replacing an expired Cosco Alpha Omega seat, which has served us well! (basically the same, but new one has improvements to adjusting the harness and more comfy padding)
That sounds like a good decision, esp as she's used to the seat's predecessor. She's comfortable, you're getting a great install, it's easy to get her in and out--I think that style seat is good for your needs.
Just as an fyi, despite the labeling on the seat, please don't think it's good to use this seat as a booster later. The belt fit is very poor and it doesn't last that long as a booster either. I know you're not intending on boostering her, b/c she really needs the harness, but in case of another child using the seat later, I wanted you to know.
For future knowledge, no matter what the stated height limit is on a seat, it may still be outgrown by height earlier than that by some kids. Kids who are longer in the torso may outgrow a seat w/a supposed height limit of 52" when they are only 47" tall (when shoulders go over the top slots). In your dd's case, that's not nearly as likely, but in my kid's case, it's always true. Our Britax seats supposedly go to 49" tall (and have 16.5-17.5" top slots), yet my 6.5 yr old is just now 46" tall, and he's been out of it for over a year! Our now 3.5 yr old likely won't make it to 4.5..and he's 39" right now.
At some point, she may outgrow this seat, too. From what you've said, I'd expect that will be a very long time from now. The Safety First Complete Air would be a similar style, fit, and install and will accommodate a taller child.
Good luck--glad to hear that you found a good solution!
(And I wanted to say...we lost a baby--miscarriage, 2 years ago--a girl w/2 trisomies...2 & 16, I think...they said it was unusual to have 2 trisomies...)
henrietta