Why is NAD+ free to diffuse within the mitochondrion whereas FAD+ is not ? What biochemical properties cause this difference ?
As explained at the Wikipedia page for succinate dehydrogenase the FAD is covalently attached to one of the subunits of the protein. It is the first carrier in a short intramolecular redox chain involving Fe-S clusters, and possibly a haem, before the electrons derived from succinate are passed on to ubiquinone and on down the respiratory chain.
As you say, this contrasts with NAD which is a diffusible coenzyme.