1. you can mobilze only a number of units up to the income value of that territory. does this include land units only? or air and sea units as well?
It includes all units.
2. can defending planes retreat ?
No. The only thing close to a retreat for defending planes occurs when their carrier is sunk out from under them. If the fighters survive the combat, either because the attacker retreats or is destroyed, they must find a place to land within one space or be lost.