I worked for two airlines but pretty much, the whole UM deal is universal.
-Some airports allow gate-passes for parents to accompany their children to the gate. This will depend on the airport authority. Ask the airline.
- The child is accompanied on all portions of his voyage. If the gate-pass won't work, the child is taken by an agent to the aircraft and pre-boarded. They were directly handed over to us, with their paperwork.
-All connections are taken care of, as is immigration and Customs. We held their passports and filled out their paperwork for them. The child is taken through the "fast lane" in immigration. The agents help them get their bags and escort them outside. The child is never left alone unless just to wait in a secure room (usually with other UM's). They change terminals with an airline rep.
-If there is a delay, the airlines will keep their UM's usually in a separate room. If they, in the worst case, have to overnight, the airline will take care of the hotel and an escort will stay in the room next door.
-The contact person has to sign, even if the child is jumping up and down shouting "Daddy!" At some point, you have to show your ID and grab a pen. The absolutely will not hand over UM's without the person on the paper showing ID. Please note; if you are somehow delayed, you can't just ask someone else to collect him so plan for that. I once stayed with three UM's when their family friend was late. They were kept "airside" and not taken outside to the public until he arrived.
Yes, it is possible to fly into a connection point and have the child handed over en route. Not sure that would even apply to you and am not sure on those details. I've seen it but too long ago to be of much help.
What I can't tell you exactly is how UM's change airlines. Hopefully someone can chime in here with experience.
Now, confirm all of the above. A friend recently had a bad experience but I'm not clear as to what happened. She escorted them, then somehow they got on board with their paperwork still in hand (nothing dramatic, just procedural). So get the story straight with them.