Was Izanagi ever hinted at existing before Danzo used it? No. These things are meant to be a surprise, like Tsukuyomi, Amaterasu, Susanoo and Kagutsuchi. Why would Kishimoto ever hint at it? That's just foolish to think about.It's not it's existence that bothers me. The fact that Itachi had never mentioned or hinted at having it as well as it being that one sharringan visual genjutsu that doesn't require eye contact(unlike all visual gen) that happened to be the perfect solution for that fight.
It also makes sense for it to have the same requirement as Izangi which was previously Uchiha eye and senju dna, considering that they come as a pair as you mentioned. Also the whole idea of Uchiha spamming Izanagi enough for them to create Izanami according to Itachi is also far fetched and poorly thought out when they only have two eyes.
Just seemed like a quick solution for his problem at the time which was Kabuto was stopping everything the Uchiha brothers could do.
Now you have to understand that Itachi was talking about a war torn era centuries ago before their own existence. When the Uchiha clan was huge, when bloody wars raged all over the world, people did anything and everything to win, the Uchiha were no exception, of course they would abuse their Dōjutsu if there was a way for them to.
Izanagi was the watered down version of The Sage's Banbatsu Sōzō, that existed in Uchiha's eyes through their genes. With it they could change their own fate in battle. But over time, they started abusing this, more and more Uchiha's were looking for the easy way out instead of honing and developing their skills, they started relying on Izanagi, one after the other.
Now if your entire clan has started procrastinating and become arrogant in their ability to change destiny, wouldnt you want to fix things? Save your clan members from this path of arrogance and ignorance, and make them realise their mistakes?
So Izanami, was developed as a variant of Izanagi, from Izanagi. Only those, who cannot accept their fate, who they really are, what their mistakes were in life and failed to look at themselves rationally, were targeted with Izanami, it works only on these types of people. And only those that accept their fate and their true selves, are able to end this infinite Genjutsu on the mind.
Izanagi can change destiny. Izanami decides destiny.