I'll give two examples that include seasoned CMs working the line:
AK was in a "free standing" line for character. Mom had her kids tag teaming all the other characters around. When she was next and they hadn't showed up she told me to go ahead. CM asked her why and she said she was waiting on her kids to return. He told her to leave the line and go to the back ... and join in when your kids arrive. She got ugly. He told her multiple times you are not to enter the line until your entire party is together and you stay in line together. She got beyond ugly leaning in with awful words and stormed off. I had a feeling it wasn't going to end there so after we were done I went same direction. She was talking to a manager that had been called over as she was demanding he be fired. I then talked to the manager to relay the conversation and manager said "CM was correct, the entire party must be together in the line, she couldn't hold a spot".............
MVMCP DS1 really wanted to meet Sandy Claus. It was the end of the night and he was done. He is obviously disabled and we know that
DAS doesn't work. I asked the seasoned CM if it was at all possible for my other two to stand in the line (which was "short" aka 45 minutes) for him and have him join on the last turn. He said he is not supposed to let anyone join the line later but given his disability ... if we sat right there where he could see us and not go off to do other things, he would allow him to join them. So we sat right there and waited.
Two CMs I would trust and a manager all said the same thing. Disney policy is no.
Also I noticed this last week more signs up that said all parties must be together when they enter a line. I don't remember seeing them before but I assume it's to address a problem. Imagine if you have been standing there a long time and every other group in front of you is suddenly double and triple the size. This is especially a problem at the specialty meets at the parties where time is limited already to enjoy. Pick what is important and get in line for it.