Gee, it's been a while since I read this, so I don't remember it with detail. But the main theme of the story is motherhood. A mother would do anything to save her child, even go back in time. In real life, there are several accountings of mothers lifting cars to pull their babies out after traffic accidents (I guess adrenaline does that). In Balancing the Equation, the author spices it up with some racial themes.
I guess you felt confused with the black hole because you were expecting a plausible, scientific answer to the story's problem. Maybe you were looking for hard science, but that doesn't seem to be what the author was aiming for. Instead, she aimed to tell the story of a black mother saving her child, and used soft science to do that. And she was right, for using hard science would attract too much attention to the science and not to story's theme, and it would occupy much more space. Soft magic, on the other hand, is often used for metaphores and symbolism, and it doesn't need to be further explained nor fully understood in order to work.
I don't recall the visions and the horror of the entire human race dying out, but I'd say that's the dimensions and timelines intermingling as the MC was messing them up. Also, haven't you heard that, if you find yourself from another timeline and get in contact, that would create a paradox that would destroy the whole universe?