An oral defense is ultimately a meeting, with the same weaknesses as other types of meetings. In particular, if new information comes up in a meeting, you cannot reasonably expect to get useful responses from the other participants. First impressions maybe, and plausible-sounding bullshit from those prone to generating that. If you want anything more, you need to provide the information in advance or schedule another meeting later.

Oral defenses in academia are largely rituals. If a student fails their defense, it's almost always the supervisor's fault. The supervisor is supposed to be the primary quality control. With their regular meetings with the student over the years, they should be able to tell whether the student has achieved sufficient understanding and contributed enough to graduate. If the supervisor thinks the student is ready to defend, the reasonable expectation is that the student will pass and graduate. The defense is mostly there to let the other examiners validate the supervisor's judgment.