What I am talking about is not theory. There is no way that brokers will freeze somebody's account just because it's been traded by someone else, unless it's unauthorized.
That is not true. Authorization can also be verbal. But if you mean having all the papers, that's not what the OP was talking about. He said:
Suppose a family member says:
"Here's my login info, make some trades in my personal brokerage account for me. Purely as a favor with absolutely no compensation at all and no liability."
That did and will get the account frozen. I'm talking about practical cases, what happened to me. Not theory.