Layering animation is like controlling water flow direction that disobeys gravity
For example if you apply the layer on the Spine bone the the animation flow will occur starting at the Spline and flow upwards to the head (ect) and would included the props parent.
If you apply the layer at the root bone then the flow will be up through the torso and down through the legs as both directions are with in the layer flow. Assuming that the prop is attached to the rig then there is no need for a layer unless the unique animation needs to be unique assuming that the animation is absolute. On the other hand if the prop needs a unique set of animations you will need to add an additional layer to the migration path “after” the first Spine layer.
The only time you would need to branch off of the same layer is if two separate components share the same animation base that splits off in two different pathways. Left and right eye joints for example so in this case your example above would work as the eyes are generally child bones of the head. As it’s set up now I don’t see it working properly as your trying to layer the prop with in the same laying as the splin joint so the prop is already included in the water stream so to speak and would need it’s own layer