Can you cook with white pumpkin?
You sure can. Just as you can cook most things in the squash family.
Yes. It should not be problem.
Yes. It's often used in Indian and Asian cuisine. It's not very sweet and it's rather squashy tasting, if that makes sense.
Sure. Pumpkin is very popular here in South Africa, but almost all are either white or blue skinned.
The orange ones are grown almost exclusively to sell to North Americans around Halloween. From what I hear, there's pretty much one guy who grows them in the Joburg/Pretoria area.