OOC: Yes, it has to be activated.
Oh, I see your point about protecting against charms. I think protection from evil spell does not protect against charm monster, because it only protects against effects that grant the caster ongoing control. In the text for charm person, it says that you do not gain control of the recipient; it just believes you are a very good friend and ally.
Although the above paragraph might be open to debate, it does not matter in this case. When Minx cast mirror image and prayer, she clearly did not want a lesser globe of invulnerability effect make the former spells useless, so the ability must not have been activated.
Edit: Infiniti, while hanging laundry (hanging laundry is perfect for when you need to think), I remembered that you do not have the Fiend Folio, which got me to wondering where you were getting your information. I looked up angels in the Monster Manual, and saw that the protective aura does not have to be activated, unlike the protective aura as described in the Fiend Folio.
My ruling still stands, due to Minx having used spells that would not work in the aura.
This discrepancy should be addressed, however. The protective aura as used in the Monster Manual should probably be the default, although it is silly that devas have spell-like abilities that they would not be able to use, unless they first used dispel magic on themselves to eliminate the aura.