If I am boarding after elite boarding and the regular line is very long, I will use the red carpet.
If there are only a few people in the regular line, I will just queue up with them - no big deal to wait my turn.
I would never tell someone in the red carpet line that they don't belong there. Not my business or my job.
And I don't undo the strap. I'll wait until the GA has a chance to acknowledge me and take care of it.