Soarin doesn't have wheelchairs, but several rides do have them for guests to transfer to for ride entry when an ECV can't go through. You may want get a wheelchair for Epcot and to ask guest relations for a GAC that says she can't do stairs for when you tour the other parks. It is also possible they would allow her to use the wheelchair entrance and transfer her to a wheelchair at attractions that have them (That may be up to the CM's at each ride, requiring you to ask each time.) I'm thinking particularly of Splash Mountain, Big Thunder Mountain Railroad, and Pirates all which have difficult queuing areas for her. Just explain her abilities and disability as you did here. I've seen many people with GAC's allowing them to use the "alternate entrance" or wheelchair entrance who were not in chairs but were walking. It doesn't hurt to ask.
In the end, she may need the chair, but there is something to be said for having independence, so if she can walk well and really prefer to avoid a chair as much as possible, I would ask if Guest Services could help. I really think that the key is to ask the CM for their help rather than saying that you need a GAC with X, Y, and Z and to keep the requests reasonable. What you're asking sounds very reasonable to me. They may say to get a wheelchair, but I think it could go either way. Good luck to you!