Alex,
Here are some words of wisdom from someone who has lived through the variable planetary gear train design and implementation cycle.
1. You do not get ANY added output torque from the ring gear motor ... zero, nada, nothing. The ring gear has to react against the input torque of the sun gear and adds nothing to the output torque.
2. The ring gear motor only changes the speed ratio. Designing it to not need motor torque at 0 RPM is a good idea, but the Denso motor worm gear may not be able to handle the load.
3. When sizing the gears, make sure when the ring gear is spinning in the opposite direction of the sun gear that the robot still moves forward. If you do not do this then any amount of force can stop you. However, you can pretty much stop any amount of force pushing you.
4. When designing your gear ratio's, there are three basic areas that the CCT gives you: Ring gear in reverse, ring gear at 0, and ring gear forward. Design your normal operating conditions at ring gear at 0 speed.
5. Read the paper that I attached here:
http://www.chiefdelphi.com/media/papers/1361
-Paul