Alright, final product is a combination of the partial solution and a workaround. When you touch a block, it immediately switches to 50% opacity and disables itself as a shadow surface, but also doesn't render before shadows again until after it's back at 100%. The shadows disappear when the crumbling blocks are touched then, but appear as soon as they're whole again, and all transparencies look good.
So it's about 80% of what I hoped for, and it looks good. Thanks guys!