I think this is a really bad idea unless it is also made very clear that this is expected not mandated and mangers are allowed to deviate from the expectation. In any large enough organization there will be teams where most people are genuinely in the top two ranking slots and others where most people are in the bottom two. Forcing a specific distribution penalises the better teams. Of course, this assumes that managers are evaluating fairly across teams. I used to work for IBM when they used forced distributions and stack ranking, and it was sometimes said that the smart thing to do was to move to a team of under-achievers so your manager would be forced to give you the top rating.

Ah, you addressed this in Point 8, but I think what I wrote above is still relevant.