I use my Dexcom G5 directly with my iPhone. In terms of being on the plane, all you have to do is switch it to airplane mode, and then manually turn back on the bluetooth, and it works the whole time just fine.
In terms of security, I go through the scanners with my sensor/transmitter on. I used to mention it, but I've stopped since not once has it gotten flagged/have I had to show the letter I carry or get a pat down. I wear it on my lower stomach; I don't know if other sites would be more noticeable. Also, the transmitter has continued to work just fine.
Back when I used the Dexcom receiver, I first used to pull out my letter to avoid putting it through the receiver, then decided it was silly and put it through anyway with no harm whatsoever. Dexcom is just covering their a** because they haven't explicitly tested that (and don't want to be liable if something goes wrong), but there's no reason to think it would be a problem. Your comfort level with that may vary though.