Dylan Kimbrell

You can do this with two sets of vellum glue constraints. "Intra cluster" constraints to hold the grains within the clusters together and "inter cluster" constraints hold the different clusters together near their borders. The trick is to use the cluster attribute in the "Cluster Attrib" parameter on the intra node and in the "Piece Attrib" parameter on the inter node.

Put a cache node in your chain and motion blur will work.

Setting the exclusion rule on the bark light, in addition to whatever other rules you've already set, stops it from shining light on the box object on my machine. If you're getting a different result, perhaps it's a bug. To stop casting shadows, you have to create a separate shadow rule.