United airlines baggage charge, will i be denied checked baggage?

I recently booked an international flight from Edinburgh to Newark, without realising there's a 60 dollar baggage charge for each way, i'm happy to pay this upon checking in

My concern is that at the airport they may possibly tell me there isn't room for my suitcase, has this ever been the case before? I can't even call the airline due to all their offices being based in the US

Can you switch your flight with another airliner?

My mother works for American Airlines so I get free flights. I'm going back home to Atlanta but my flight has a long lay over in Dallas for 6 hours. It's a 6:00 am flight that's on delta. Mom has flown delta before coming home from work because one of the flights was full or whatever. I kinda already know the answer but I was wondering because I'm in south bend Indiana. I doubt that flight will be full. I have my booking info would I even have a chance to maybe be able to get on that flight?

How long can you stay at an airport for after you Land?

So I'm heading to Los Angeles (LAX) and I land at 11Am but I can't check into my Airbnb until 3pm. I can just sit at the airport for like a hour or two just to kill some time before I need to check in? Or is that a security threat?

How do i find my plane at the alanta georgia fast?

I'm 16 and going on the plane for the first time. I'm going alone. I'm from alabama going to virginia. I need to take 2 planes to get there. The first plane is taking me to atanta i have 45 minutes from when my 1st plane land and when my 2nd plane leaves. From what i heard about my cousin it tooked 30 minutes just to get in the airport. Which could leave me at a possible 15 minutes! I heard atlanta's airport is one of the biggest airports on the country. I have no i idea how i can find where am i spose to be at in that length of time with the size, not even being in a regular size airport before. So how am i spose to do his

Added (1). How do i find my plane at the alanta georgia airport fast?