I am concerned she would feel uncomfortable doing RSR single rider at 9. If I knew we would be in the same car this would be helpful but this typically doesn't happen. What are others experiences of doing single rider on RSR with children under 10?
I would ride together first, then designate a specific waiting area. Be very clear about this, but make sure it's not in an area where the CMs will shoo her along. I ran into problems when my son was 12 (before he got a phone) because we got sent to different sides of the Matterhorn, and he was waiting in the exit path. The two sides don't exit down the same path, so it took me a bit to find him.
Also, I would ride first, so I can hopefully be the one waiting at the end. Occasionally, the second person gets caught in a delay that the first doesn't, but then she'll just be in line or sitting in her ride vehicle. Annoyed, but safe.
Ultimately, it depends on how comfortable she is. My daughter would not have been comfortable at 9, but my son would have (we did it with him at WDW at 8, and he was happy as a clam)
I think once my daughter rides it once, she'll be so thrilled that she can ride again without a long wait that she'll be fine. That said, my sister is coming with us, so we can sandwich her between us. She'll have to save her bravery for the Incredicoaster, because my sister doesn't do rollercoasters.
All that said, being in the same car on RSR isn't that uncommon. They put groups of 4 in separate lanes, so it's actually quite common for them to send 2 single riders in after them. I think I actually got to sit next to my kid on one ride, because the family in front of me had two kids who wanted to sit next to mom or something.