Another thing I should maybe mention, it is perfectly feasible to just have Entities cache the 1 to 8 Nodes of the Octree that they are inside of. Especially useful for Collision Checks and such. You dont have to traverse the entire damn Tree just for changing minor things.
I am aware that you will probably have to update the “List of contained Blocks” of all Nodes up to the Root Node, but if you are placing a Wall of Cobblestone Blocks, you wont even need to go all the way up to Root, just up to whatever Node already has Cobble in it.