About Devil Trigger, it's not "air only" as far as I know, I believe that you have the regular devil trigger meter, but as soon as you go to an air combo, the Devil Trigger will last as long as you stay there. If you touch the ground it's over, so having Devil Trigger for as long as you can will be some kind of skill challenge.
And no, it won't be as simple as launch -> pull yourself (over and over again) becase as indicated in some trailer (I believe TGS Extended one) and The Secret gameplay, there is "a rooftop", and if you get there Dante will automatically launch a charged punch like move to the ground, an area attack. So if you just launch and pull yourself like 3 times you will basically do like 6 hits and lose your Devil Trigger. Jump Cancelling would be much more suited to stay longer in the air.