On the knee walker thing too, it might be too soon after breaking it and there's the possibility of it getting banged around, which needless to say would not be comfortable. So soon after the injury, wheelchair is probably better if for no other reason then to keep it stable.
Make sure, even though you're in WDW, that you still have her ice it. Otherwise, it will swell under the cast and it will not feel good.
Generally, they wouldn't give you a DAS for the wheelchair, but you can always ask. The worse thing they can say is no. I just heard back from a family friend who has MS and needed a wheelchair, and she was able to get one, so you never know. It might be good to go to guest relations just to get some guidance anyway. AK has a designated area near the front of the park with CMs who do nothing but address Disability-related issues.
As stated above, there are rides that have wheelchair-specific entrances (Space Mountain; there are stairs in Standby) and cars (TSM comes to mind). My thought would be that if she doesn't have to transfer, let her stay where she is. Again, so soon after the injury, keeping it stable is best, so not transferring if she doesn't have to is probably better.
Bring something to wrap the foot in for Splash Mountain. You don't want the cast getting wet. I would also bring something to wrap it in in case it happens to rain, though you are thankfully going during the dryer season.
The good thing is that you should only need the bus for MK, AK, and DS. You can walk to Epcot. I would not walk to DHS; take the boat.
