Interesting. I'm not sure whether or not it's better at shielding, because we're told that people break in to Azkaban to shield the inmates so that they might have regular non-nightmare dreams, or just a half-day of patronus time. So we know that just one typical patronus is strong enough to protect people from the worst effects of a Dementor for 12 hours.

I don't think we know enough about the defenses of Azkaban to say at what point the typical rescue operation would fail. But when we're witnessing the aurors in the command center, I find it interesting that only attempts to relieve the pain of being in Azkaban through patronus-presence are brought up (in the bit about bribes), not escape attempts. Perhaps it has to do with the "perfect crime" logic.

As to what the actual purpose was in this whole excursion, I have no idea.

I'm not sure intentional failure is the only explanation. It could be some weird bonding experience. Maybe Quirrel always dreamed of raiding wizarding prisons, pulling off bank heists, and taking over the world with his son. Chapter 55: "Adoption Papers"

I think from the duel that we can infer that Quirrel didn't expect to lose, even in a one-sided fight against a team of aurors. He was just playing games when it was one-on-one. Maybe he used the killing curse because he was (overly) confident that Harry was committed to trusting him completely with regards to this mission and didn't expect to be blocked.

Maybe chapter 55 will answer all of our questions. Ha. Haha.