Why wouldn't the Warden just give the order to keep the island at elevated defenses "until ordered otherwise" ... Then the island is defended even if the Warden dies, because the order was given by a "very-alive Warden."
Apparently that didn't happen, yes, some defenses continued even without a Warden, but the island was vulnerable. Because many of the island's defenses that only a Warden could control were down. What underscores that, is at the beginning of Skin Game one of the things Harry said he had been doing that past year was learning what those defenses were and restoring them.
Instantly? Likely not, if we're to judge by Mab's reaction.
Yes likely, page502 Cold Days, Mab had just taken the gun back from Harry and was pointing it at him, bolding mine;
"I," Mab said coolly, "I am not yourservant, Dresden. "You are mine."
"Demonreach," I said. "If our guest pulls that trigger, take her below and keep her there."
The guardian spirit's vast shadow fell over us even though there was nothing actually casting it, and Mab's eyes widened.
Wide eyes mean one of two things or both, 1] surprise, 2] fear.. In other words it was an "oh sh#t!" moment for Mab. Mab knows what the island could do and knew in that instant that Harry had the power and wasn't afraid to order Alfred into action, she also knew she was doomed if she made any move against Harry.. So...
Harry goes on to warn Mab to be gentle with Molly or she will answer to him. Mab backs off and says her usual about how pleased she was to finally have a strong knight. But make no mistake, Alfred was in motion to carry out Harry's orders if Mab had pulled the trigger, and he would have put her in the slammer.. When Alfred did it, most likely Harry would already have been dead.