Knocking someone out safely isn't cheap. There's a reason anesthesiologists are so highly paid. Just ask the hostages from Dubrovka Theater [0] how improvising an anesthetic gas can go (spoiler: you'll need a medium/ouija board).
In theory yes because you can use the space at the top that would otherwise be above people's heads. No guarantee they get to their destination alive though.
Once you include space to get people to the restroom though and to allow them to breath during the flight I think you get a higher density while standing vs laying down.
Only a new airplane. Most planes are designed for a lot of air and couldn't fly with as many people as can physically fit inside. Cargo airplanes carefully watch this factor. As a passenger I've been on airplanes that took off with empty seats even though there were people on standby wanting to get on because with the weather they couldn't fly a full plane.
You and I should talk :) I've been thinking of this ever since seeing the movie "The Incredible Burt Wonderstone". He uses a sleeping gas on his audience packs them into a moving truck and "magically" transports his audience to a new location. Tada! Basically same idea, but substitute moving truck with jet.
That's still way too inefficient, it leaves so many gaps and barely tiles the space. As soon as we get our hands on full reconstruction, you can bet the airlines will require everyone to be ground into a slurry and pumped into the fuselage like a huge tank, and get reassembled at the destination.
