Usually, If they are under 13, a chaperone will be provided. They will walk the girl to the airplane, check on them every now and then, and walk them out. Almost all of them do. Check their websites and go to customer service. Or when you go to the airport, ask the people at the front desk. Have a safe flight!
I don't believe any airline will let her travel alone. And if you buy her an adult ticket, when she checks in and they see that she is a child, they will require you to pay the fee for the unaccompanied minor child. It is actually a good idea in case there is a problem with one of the connecting flights.
Most of them if you inform them prior to departure. Especially if its first class.
An adult may have to be at each gate to pick them up and they might need to escorted by security.
Call the airline companies.
None. United and southwest allow 12 year ODs to fly as adults. All others require at least 15. I have checked this several times on U. S. airlines...last time 2 weeks ago.