The basic problem with that is that while Tusk mamoswine can't beat leadape-done-right, neither can any other mamoswine. Ape will always be faster and without the lead effect the sash will be broken by stealth rocks. So....the fact that ape is the perecft counter to any mamoswine isnt a fatal flaw to this set. It's neutral to the others in that one reguard.
What I offer is a Mamoswine that can easily fit many niches and is one of the most versitile pokemon in the heavily comparmentalized OU tier.