skeletonNode->setScale is a cocos2d-x method for scaling a scene graph node. The scale in the CCSkeleton ctor is for setting the scale on SkeletonJson, the JSON loader. It scales the skeleton, animations, and attachments. This allows you to design in the Spine editor at one unit scale, and use a different unit scale at runtime.
The attachment width, scaleX and other fields do not change at runtime. These are the attachment offset SRT relative to the bone, and the attachment size. The attachment is first positioned, rotated, and scaled based on the attachment offset. Then the bone's world SRT is applied to these points. Note that if the attachment is not aligned to the bone coordinate system that the attachment quad can be skewed from a rectangle to a rhomboid if non-uniform scaling is used. If you are using the box2d body's SRT, don't use non-uniform scaling in your setup pose or animations.
TLDR; use the attachment offset SRT to setup your initial shape. Use the bone SRT for the box2d body.