Would the shadow retain the look and form of the dragon?
If so, does it still get its greater reach with a "bite"?
Would it retain its defensive abilities while gaining the Shadow's?
Anything else of particular note?
Should retain the look and form of a dragon, it is after all the dragons shadow.
Yes.
Probably not. It should only get the abilities of a basic shadow as per the spell. A players special abilities don't transfer over.
If I'm wrong on that I would say it certainly loses any defensive abilities wholly dependent on it's physical form such as natural armour and elemental immunity.This template might give some ideas as to what should transfer over.