Your info about Ruin III is incorrect. Once you acquire Ruin III, it automatically replaces Ruin I. So there will be no option to spam Ruin I in the interest of MP preservation.

Also, I saw in the videos that when Futher Ruin procs, it automatically changes the icon of Ruin III into Ruin IV. It's an automatic thing that we really don't even have to think about, which is nice.