Doesn't sound clear to me. "May" does not mean "shall". Therefore succession would be challenged. Count on it.
LOL...nice try. The 20th Amendment gives Congress full powers to choose a line of succession. It did. The only way this is litigated is if there's some constitutional question over the application of the law, which seems extremely doubtful given the separation of powers.
Therefore succession would be challenged. Count on it.
Don't worry, I'm not.