Must say I have never liked heartwood in my bats. Just to clear up a misunderstanding, heartwood is harder but in a cricket bat not more durable. It has a lower moisture content and therefore more prone to splitting as it cannot deform as readily.
If people are saying that willow with lower moisture content does not last as long then heartwood bat should in theory have the shortest design life.
I don't think the number of grains, thickness of the edges etc have any bearing on the quality or size of a bats middle. However, the moisture content of the timber will have an influence on its life.