Yes and no.

If the thunderhawk landed during its ground attack action, it becomes a ground unit and so behaves like other forms of flak. So it can attack aircraft who moved within its range and arcs at some point within their action.

If the thunderhawk does not land, it fires defensive AA if enemy aircraft end their move within range and arc.

In the circumstances you describe, it makes no difference if the attacking aircraft are on intercept or ground attack orders, or if the aircraft lands. In all cases AA attacks are made on the basis of approach moves.

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Not quite.

When aircraft move onto the battlefield,

• Only Fighters and Fighter-bombers can use their AA to attack other aircraft.
  
• All aircraft may use their AA defensively (when they are attacked)

Later in the turn,
• all aircraft that are flying may shoot at enemy aircraft that *end* their movement in arc and range.
• If the air transport actually landed, it becomes a ground unit so may shoot at enemy aircraft that *move through* their arc of fire in range.