This rig setup mostly works in Spine right now, the only problem is that the art/attachment layers don't understand the concept of a control bone or the transform constraint, so draw order is static and also art frames have to be manually swapped out.
You are right! This is indeed an issue for which Spine should provide a useful solution.
Thanks to the addition of folders for draw order in 4.2, it is now easier than older versions to create animations that require changing the draw order of many slots, such as rotational animations. (For example, in the older version, if you wanted the right arm to appear before the body, you would have to move all the slots for the rightarm, right hand, right hand ring, bracelet on the right wrist, etc., but by moving the "right arm" folder, you can manage them.) However, it would be nice to be able to save frequently used poses as a kind of preset and use them as needed during animation, not managed by keys as it is now. With the current management method, too many timelines are listed on the Dopesheet view, and because timelines cannot be locked, keys may be deleted or be moved at any time while working. For example, when offsetting, it often happens that the keys for changing the direction of the character placed at frame 0 are moved and the loop is broken. I know this is just my fault and can be avoided if I'm careful, but I can't help but wish I could control the poses more strictly.
I think the best approach to improving such cases may still be debatable, but I can at least very much agree with what you feel is the problem itself. Thanks a lot for your detailed feedback!