You can keep your carry on under the seat if it fits and you don’t have to wear your seatbelt if the light is off. Yes on the seatbelt for landing but they turn the seatbelt light on.
You should keep your seat belt fastened unless you are getting up. You never know when turbulence may occur, bouncing your head and neck on the ceiling. For the UMPTEENTH time, this is for your SAFETY! And while your neighbors seated around you may be rooting for you to get your just desserts, they also would not be happy if you landed on them.
Funny how people don’t understand that the FAA rules and plane rules are the law and they can get arrested for not following the rules, or get on the no fly list. lol
I can imagine the untold number of flight crews that would absolutely LOVE to have this option available! If it was I can guarantee the number of Karen's and Ken's would dramatically decrease on flights.
I pay to check my luggage so my backpack can go in overhead bins and keep my foot space free for my bum knee. But I am always asked to put it under the seat in front of me so people who did NOT pay can have that space.