Pairing doesn't do this. You just attack once with the stat bonus from the pairing. HOWEVER, I'm still unsure if you Pair Up and then have an adjacent unit if you still get the second +N attack, my guess is no.
No. You can only one be with one unit at a time so you can't have adjacent units joining your attacks if you're paired up.
My question: When paired up and you are attacked, will both characters counter attack that are paired, or just the main unit?
The main unit is the only one that will counterattack. The only thing your support can do is jump in and block the damage.