I will restructure both of your questions into two use-cases:
1) If you are talking about the initial structure:
I suggest asking an interviewer something like: "Is this structure good to move forward with? Am I missing something or is there anything I should prioritize?" I recommend doing that both in candidate- and interviewer-led cases.
2) If you are talking about a structure you make while solving a case:
You simply take 30 seconds and make a new structure. It's important! Don't take 30 seconds and come up with 1 hypothesis (random guess). When you take additional time - always draw a new structure and present it to the interviewer.