attacked units are dealing dmg first. If you attack, the defendant will it your units with "Defence" power, then your damaged units will attack him with "offence" stat lowered by the dmg they taken. But be careful, because patrolling units will attack your planes if they will be in their range while they end their patrolling circle, so they will dmg your's units twice if you mistime the attack

How do you know when an opponent's patrol cycle timer will run out? For that matter, your own patrol cycle timer?

Click on the patrolling stack and you will get a timer that counts down to the next time the patrol cycle ends in the Unit UI.

Alternatively, when you zoom in, there's a blue patrol bar that fills up and resets every time the patrol cycle ends, though it doesn't tell exact time.

Unfortunately, their lower defence vs. air has no effect at all. Due to a bug, planes receive offensive damage when they attack instead of defensive. As the attacker gets hit first and then does his own damage (so already lowered by the damage he received), and both sides use offensive values, this ironically means the defender is in advantage. However this may get fixed at some point, we'll see.
If your patrol timer runs out and an enemy wing's patrol circle overlaps with yours, you will deal 1/2 offensive damage to that enemy wing. If the centre of their patrol circle (so their position) is within your own circle you will also receive the offensive damage from them back.