Cold Wax Medium as Varnish

You can, but realize that wax actually attracts and holds dust. Additionally, if the painting surface is at all absorbent, the wax will probably never be removable. Finally, the product you mention does contain a small amount of alkyd binder. If you like the surface it creates, I would suggest that you make your own wax paste using odorless mineral spirits and bleached beeswax alone. You should never directly heat anything containing an organic solvent or alcohol. This is best done by melting the beeswax in a disposable container. Then remove the molten wax to a completely different room or outside and then add the solvent to the molten wax. Continue stirring until it is evenly combined.
You will need to experiment to determine the ratio you prefer. Thankfully, beeswax does not yellow over time, the residual wax will not become disfiguring with age.