Can land and sea units adjacent to the territory or sea zone being attacked ( also units range permitting) move in and help defend?
First of all, a defender can't move sea units into the Caucasus. (An attacker can fire a one-shot bombardment from a battleship in conjunction with a land attack.)

The second thing is that a defender can't move units during another player's attack portion of the turn. A defender may move defenders into a territory during the portion of the turn when s/he is entitled to attack.