Is Jetman flying above other aircraft allowed? How it is possible?
Yes.  This is part of a series of promotional stunts, "adverts", Emirates are undertaking.  Aircraft can fly in formation, all other rules taken into account, if the captains agree.
By flying above the aircraft.  As the article states, it was meticulously planned for safety (after all, an accident would be disatrous PR), and the Jetman flyers would know exactly where not to go.
Yes.  The video shows that!  The aircraft will likely be flying at slow speed.  As you can see, the flaps are extended.  I'm guessing about 160 kts.  I would also imagine the flyers will have dived down from above to pick up speed.