Hi AK,
Invigorator lights, with shadows enabled, allow shadows to be cast from one Invigorator object to another, so long as they’re in the same scene. As you’ve discovered, those shadows do not extend to other AE layers.
As you know, AE lights can create shadows for 3d layers, if those the lights and layers are shadow-enabled.
In your situation, you could turn the Invigorator layer into a 3d layer and enable cast and receive shadows. You would then would then be able to generate the shadows you want. However, you have to be careful to keep the Invig layer oriented toward the camera at the correct distance, or the 3d illusion will be broken.
Does this help?
Jon