The analogy between Marcone's vault and Pell's theater work's this way for me. Without Marcone's vault Harry couldn't get to Hade's realm period. I would assume, were I Jim and I was writing it, that accessing the vicinity of Arctis Tor would be similar in nature in terms of difficulty,to the difficulty in accessing Hades. So once the theater was prepped anyone could open the portal, but without the theater no human could. YMMV. That's how I envision it.
Your third point. I don't suggest that it happened this way in Jim's mind, but think of a slow zone surrounding Arctis Tor where time slows down. So if your coming to Arctis Tor to attack, time will slow for you while Mab in her fortress proceeds at normal time. She will appear to you to be racing, while to her you will look like a turtle. It's almost a perfect defense.
However if access to Arctis Tor is restricted and Mab ends up in control of the crossing point at Pell's theater than you don't need a guard at all, you always have time to yell for help. The assault on Pell appears to have accomplished precisely one thing. It kept him away from his theater and kept it closed during the events of Proven Guilty. As to why Mab might do something like that, any theory is as good as mine, since I don't have a coherent one. WOJ is that Mab had Molly brought to the fortress. Obviously she wanted Harry there and she has shown a willingness to accept collateral damage to accomplish her goals. Other than that I'm more or less clueless.
I don't think I covered all your points, however I am currently out of ideas. Thanks for the back and forth. It helps me quite a bit.