I could definitely believe that Injustice is a world within the dark multiverse. It makes sense; if the failed worlds of the Metal Knights were nightmarish reflections of Bruce's fears there's no reason there wouldn't be worlds that reflected Clark's. And a world where he's weak enough, blind enough, and selfish enough to become a tyrant after Lois' death must certainly be one of Clark's greatest fears.
Whether it's like an immune system......it's an interesting idea.....but does it fit? Is the dark multiverse "alive" enough to do that? I mean, the "dark multiverse" is just where worlds go when they can't find a home in the Orrery of Worlds. They sink, break apart, and get recycled by the World Forge (when bat-demons aren't screwing with things anyway). The dm is just a recycling bin. And if it is cognizant enough to respond, it was corrupted by Barbatos...so would it recognize the Metal Knights as a threat?
Perhaps Injustice could have been made by the Forger in case what's left the Metal Knights or Barbatos return. Ha, it's kinda funny to think that Hawkman is partially responsible for the Injustice Universe.
I dunno, but I do know that I'd be more than fine with using the dark multiverse as a bridge there; let's get a digital first mini where the main continuity meets Injustice. Or hell, go for broke and also bring in Batman 66, the Arrowverse, and Golden Age Earth-2. "Digital Crisis!"