I've never done a chocolate beer but found your recipe interesting so thought I'd provide my observations/2 cents worth.
Cascade is an interesting choice. I haven't researched a lot of dry stout recipes to know what to expect here, but I suppose since it's the bittering hop maybe it's not such a big decision anyway. (Initially, I was thinking that the grapefruit aroma might clash with your cocoa nibs, but I'm rethinking that now). I think you are right to avoid an aroma hop. Let the aroma of the cocoa come thru.
I'm not familiar with cocoa nibs. Are they essentially baker's chocolate? If so, I've read from Charlie P or others that there is some oil that can form when boiled and will separate out. Among other challenges, this can affect head retention negatively. Others use cocoa powder because of the oil in baker's chocolate. You might want to research that form of cocoa.
Other than that, I think it looks good. Best of luck!