Possible that if they did every single answer would be "I don't know the exact details, my team will get back to you"

He came without being subpoenaed, so not putting him under oath may have been a courtesy. Many of the congressmen thanked him for coming on his own accord.

Yeah. I'd even go further and say that if you are giving testimony in Congress and answering questions, then you should automatically be under oath unless Congress explicitly waives that requirement before the session.

Perhaps this is more of a show than the real thing

The cost for them is 0, the cost for MZ is incredibly high. I don't think he would have agreed if he was under oath.