Breakage is broken merger, which doesn't merge, i.e. results in distinguishable states. Correctly implemented merger must produce indistinguishable states. If the substrate tries to implement merger, but fails, then the substrate is broken and doesn't implement computation.

>Implementation requires >= 1.

No, you require computation to work on a broken substrate, and that's an inadequate requirement. It's agreeable that your requirement isn't met and computation can't work on a broken substrate, but this doesn't imply that computation can't work on a working substrate.