I want you both to be better off. And if the insufferable prick Solas taught us anything it's that spirits suffer for being imprisoned in a way that strains at their purpose. I know you don't think this has harmed Justice, but it was never meant to have to deal with all of your humanity. This is for its sake as much as yours.
I don't personally feel that Justice would require or even want another host. He's tolerated his confinement in you because of your friendship. Outside of that, I do not think he would force himself on another. It would be, after all, unjust. My hope is that, separate from you, it would revert to a being of pure justice, rather than the vengeance it has displayed within you.
I would not put forward a plan that would kill you, whatever you might think of me. This would, in fact, be far easier if I didn't care about preserving both you and the spirit. But you have made it very clear that the spirit also needs to survive this. There are a number of ways we could manage it, some of which I am willing to discuss and others I am not as yet. However Lyrium and the Fade are not necessary in such a magic-rich environment as I have said at least a dozen times already.
As to the Blight, it was my understanding your Commander had been infected longer than you and still seemed in perfectly fine health. Given your spirit has been suppressing it for so long, it is likely that your Blight is far less advanced. That means it is not nearly as immediate an issue as you are making it out to be. Obviously, we also want to solve that issue, but we will have time to do so.
I do wish you'd had as much concern for all the people you had killed as you do for your own skin. Your idea of justice is a strange one to be sure.