Can I abandon turfs when attacking or maneuvering?

Yes. You may abandon a turf when attacking with all your troops, maneuvering troops, and of course you abandon turfs when retreating as well (with the appropriate penalty).

What happens if I play Call for Help and when I choose another player and draw a Combat Card at random from their hand—it is another Call for Help?

The effects are cumulative. If you draw another Call for Help, you may choose another player to assist you in the turf war, and draw a card from their hand as well. You will have to pay 2 Dominance Points to each player helping you.

How do the Teleport Action Cards work?

You will take all of your troops from ANY turf of your choice and move them to an EMPTY turf of the type indicated. You cannot use a Teleport Action Card to attack.

Why is there a Defensive Maneuvers Action Card?

Um… to enable you to perform defensive maneuvers.
Except for Gryphons, who have this as a faction ability, there is no way for you to easily redistribute troops between turfs you control—and this is by design.
Unicorns can defensively maneuver troops from one adjacent turf when attacked.

Further, if you find you’ve spread yourself too thin or wish to consolidate troop strength in certain areas, this Action Card enables you to achieve this.

When a turf-related event (Mirage, etc.) is drawn can you use an Attack Card to move into an empty region of that type?

No. An appropriate Teleport Action Card enables movement into a turf that cannot be attacked during a round.