I like that they give the Genie limitations but I wish I knew what their parameters were. Sometimes he either knows everything he needs to know because he’s a Genie and he was once all powerful, or he makes a book appear that knows everything when he doesn’t, but then on a rare occasion, he’s just completely stumped and his magic doesn’t give him any clues.

This is where Iago comes in. Iago, who worked with Jafar, and conceivably knows a lot of the stuff Jafar does. To a degree. Iago knows exactly who Mirage is, because he knows what Evil Incarnate is, and here’s where we get to some of the semi-religious stuff.
Evil Incarnate appears to simply exist to destroy goodness in the world. While the term is never used to my knowledge, this is a concept that compliments some tenets of Zoroastrianism quite nicely. With good and evil existing as distinct, living forces, there stood the belief that all living creatures took part in a cosmic battle between good and evil, the prize for which was all of creation. By living your life in the most good way that you can, you do your part to battle evil. For a figure like Mirage, as Evil Incarnate, it would make sense, then, that her function in life is to tip that balance in favor of evil. It is also why, in a world such as this, you can nod toward certain villains who play up to the notion of “being evil” for the sake of being evil as though that’s a thing. In this setting, it totally is.

Genie seems to remember a time when Evil Incarnate was someone else. “A hyper-intelligent wombat with a forked tongue,” to him. Things change, Iago informs him. This suggests that the mantle of Evil Incarnate is something that Mirage earned, establishing that she had a life before she took the role (which is further developed upon in “Eye of the Beholder,” hence why doing this episode and the last one was so important).
Once again, interesting questions raised. What was she like in her previous life? All to be discussed when the time comes.
Other things we know is that she’s not herself a goddess. There are other beings that appear in the series that she’s clearly subordinate to. She’s high on the totem pole when it comes to power (and would have to be considering that while she hates Genies she’s not the least bit afraid of them) but absolutely not the highest.
I can’t wait to see more.
My last question: Iago knows who she is. HOW does Iago know who she is? In what situation did Jafar come across her?
Is that where he got the snake staff from, maybe? (That could be interesting. But it was vaguely Egyptian in design.)