How can one leave internal void cavity in fused glass piece
I've never tried this, so this answer will be largely speculation based on the physical properties.

You can leave a void, but I don't think you would be able to make the void a complex or delicate shape.  There are a few ways to start with a well-defined void: engrave it into one or both mating faces of two pieces; start with three pieces and cut the shape out of the middle piece like a stencil; make the shape using a glass tube and seal the ends to trap the air, then embed the tube in a bed of glass dust and fuse the bed (this wouldn't be applicable for a 2 mm final thickness).

With all of these approaches, glass fuses by becoming a liquid and flowing.  The molten glass will flow in a way to minimize the surface area of the void.  It will fill thin areas first, forcing any trapped air into bubbles, so you will lose the shape.

You could embed a specific shape in fused glass by making the shape out of a material that melts at a higher temperature.  However, that wouldn't be a void.

The only way I can think of to have a void of a specific, desired shape is to not fuse the glass.  Use one of the methods, like mentioned above (e.g., engraving or stencil), to create the void, then bond the pieces together with clear adhesive that bonds glass.

If the goal is the appearance of fused glass, another approach would be to fuse the glass.  Then engrave the shape in the back side.

Another idea that might come close to your objective.  Create the void as in the gluing approach.  Fill the void with fine dust of colored glass.  Then fuse the glass.  Colored glass dust would melt, and it might hold the shape, and at least be a contrasting, embedded shape.