The Librarian provided the inspiration needed for Humanity to attain the Mantle. Each step that was taken was because of her guidance. What if that manipulation went one step further? What if Halsey was just her hands on way of interacting with Humanity to get them to that point.
Much like with the Born Stellar and his ancilla. Then with the Didact and his group of companions. Offer thousands of years she guides humanity towards the mantle. To confront of the man that is her husband. Provides him with the best training, armor and equipment. Then gives him a little blue female inside his head. Wasn’t ancilla of Forerunner armor seen as blue forerunner females?
There is more to it than this I just don’t really want to type it all out. Though the signs are there if you look for them.