Maybe I can help with a few of these questions. As for how planes are loaded with passengers. This is done so that the plane is properly balanced. If the airline required the first 100 passengers to sit in the rear of the plane and then work forward, it is likely that the rear of the plane would tilt to the rear and the front wheels may come off the ground. Keeping the plane well balanced is important.
If you have ever flown United, you will notice that they begin loading passengers in the front, then the rear and then the middle. This keeps the plane balanced. It also aids in flying and the pilot doesn't have to adjust the various movable parts on the outside of the plane. Say we only have 150 passengers onboard and they all sat in the back of the plane. This would change what is called the angle of attack or the AOA. In other words, the backend would sag down and the front would rise causing the pilot to probably shift the rudder, ailerons and maybe even the stabilizers. It's important that the wings stay level and balanced.
If you see a passenger with an oversize bag, I mean a really oversized bag, don't be afraid to mention it to the gate agent. That person is being a bit selfish by taking up more than just a space allowed for each passenger in the overhead bins. If the plane is fairly empty, the gate agent may not do anything about it, but with a full plane, the agent should tell the passenger they need to check the bag, which the agent can do for them at the gate. If they refuse to do anything, when you get home, write a letter or e-mail to airline and complain. I have known some passengers to get some nice perks from the airline in response to the complaint, including free tickets, additional free miles on your rewards card and even free drinks and better meal vouchers. Just remember to be nice when you write the letter.
When I fly, I am never in a hurry to deplane. I stay seated until it's my turn to get up, reach into the overhead bin, grab my bag and walk off the plane. If you don't hurry and treat it like a Black Friday sale at J.C Penny's, you'll be in a much better mood when you get into the terminal.
The woman with a baby who spent 20 minutes in the bathroom. Maybe she was breastfeeding and wanted privacy, or maybe it was something else. But she should either figure something out or not fly with a baby. Not fair to hog a bathroom shared by maybe 40 people.
If someone is in the lavatory unusually long, notify one of the flight attendants. They will knock on the door and ask if everything is OK. I never had this happen, but I know of a pilot that had a dead man in the lavatory and never knew it. I can't really remember all of the story, but I remember him telling us it was a long trip, like New York to Los Angeles. He said the person he was sitting beside told the police that he got up to go to the lavatory shortly after takeoff, so he had to be in there a good 6-8 hours.