First the island and the prison aren't the same thing. You go through a door to get into the prison from the island. From the text you can make some assumptions.
Mab has a high degree of trust from Alfred or Alfred is powerful enough to not fear Mab being inside the gates. The second is supported by Mab not touching the island until invited by Harry.
Mab made a low, disgusted sound and turned to face me. “I have heeded your summons; yet I would not enter this domain unless specifically bidden. Have I your permission to do so?” “Yes,” I said. “Yes, you do.”
Butcher, Jim. Cold Days (The Dresden Files, Book 14) (p. 501). Penguin Publishing Group. Kindle Edition.
The inmates do not get keys to the asylum. Neither Maeve or Lily understood the hazard that they were in in Cold Days.
Mab nodded her head slightly, and descended to the ground. From me, she turned to Demonreach. “I thank you for your patience and your assistance in this matter. You could have reacted differently but chose not to. I am aware of the decision. It will not be forgotten.”
Butcher, Jim. Cold Days (The Dresden Files, Book 14) (p. 501). Penguin Publishing Group. Kindle Edition.
By implication they were a moment away from a padded cell in the prison, which would cause all kinds of problems of balance in the Courts. Following that reasoning, giving Mab access, but with her afraid to use it, makes the idea of a Portal in the garden dubious. This is reinforced considering the portal opening would open both ways. Mab could have a Portal placed in the Garden, but why risk an attack through that Portal?
Rashid is the international man of mystery who goes where he wants to go, when he wants to go there. My money is on Rashid being able to create Portals anywhere, at will.
Final note. Summer Knight establishes the rule for Mab controlling mortals. She can't unless they are linked to the the Courts. See the dialog where she has Harry stab himself with a knife.