1) When you create a blendshape for the pose editor can you safely switch its deformation order? or is it always used as the last deformer? Do you need to change the order before making Interpolators or does it matter?
You can switch the deformation order without issue. If the blendShape is driving a corrective blendShape it should be the first deformer, unless you are using tangent-space blendShapes.
2)Can I save the poses I sculpt out as geometry or only as shape data?
There isn't a tool in vanilla Maya that can export all the shapes as .obj or .mb files. However you can write a script to do that, or likely find an existing one.
3)How would I bring in pre-generated corrective shapes? Is it just a matter of connecting the shapes to the blendshape in the right order is there some other trick?
It sounds like you have a mesh where the rig is posed, and you've adjusted that, and now you want to generate a blendShape that when activated by the pose reader will result in the shape you sculpted. Maya unfortunately doesn't have an easy way to do that without scripting. I would recommend using the SHAPES plugin from brave rabbit.
