I'm still in the throws of HG. I also have other high risk issues, but have been extremely blessed that LO is right on track. I was very high risk for abruption, IUGR, and other equally scary complications. Not that it's all over yet, but things are looking much better than they did the majority of my pregnancy.
I can tell you that the HG will be a major barrier to me ever doing this again. I do understand what you mean about wanting your LO to have sibs. I ask myself if it were a question of having my sibs or going through the incessant nausea/vomiting ED trips etc, would I do it? I always say yes, in my head, but it seems totally illogical.
Based on my experience, electing to go through anything this stressful is totally illogical. Your liver problem sounds pretty scary. Dialysis and ICU stay? Yikes. My advice is counseling. It seems like you need a sounding board, and I think counselors (in whatever form psy, spiritual, etc) are a great way to work through tough obstacles and major life decisions.
By the way I had to have an MRI during 3rd tri and it was pretty awful. I'm also concerned about the risks to LO from having done it. I had an MRI prior to pregnancy that wasn't nearly as uncomfortable. I really hope you don't have to have one (or more?) if you do decide to conceive. I know that sounds negative, but I thought it was worth mentioning. In the end, they can't do contrast anyway which makes for less valuable information in some cases. I would ask your MFM about that.
Good luck to you whatever you decide.