One thing that could get the sword closer to where you want is to put both the pig and the armorstand in the bat's passenger tag. Instead of stacking them on top of each other, you'll be putting them on the same layer.
You may be able to get the sword closer to the pig's "hand" by making the armorstand small and adjusting the pose of the armor stand's right arm.
If that's still not enough fidelity for your taste, another option would be to use a repeating teleport command instead of the passengers tag. Let me know if this is the route you want to take. Just keep in mind that even with this method it's not going to sync up with the pig's "hand" 100% of the time.