For a 42” waist which is better the 36-42 or the 38-44?

We recommend that you can buy a larger size, and as our waistbands can be trimmed, you can always cut off more.

How quickly can I adjust my belt when I sit down?

All you need to do is gently push the lever on the side and the lock will unlock, there are 32 ratchet notches,it provides a micro size-adjustment of 0.25",which can follow the variation of your waist precisely;

Will the belt set off metal detectors? Are they TSA approved?

Yes, they are subject to setting off metal detectors and are not TSA approved. We advise you to remove your belt and wallet before going through the metal detector.