This drawing is fairly accurate as to how the distillery process at BT actually operates. As far as the comment:
3. It does seem odd that is shows grain going together and then separating. I think we should remember this is a "typical" schematic rather than an actual drawing of BT.
I think you're misreading it...it's just showing that the pipe goes from the two tubs, joins up, and goes to all three cookers. There's no separation. As to why they aren't mixed in the fermentors...like everything there are many reasons one could come up with, but the economic reason is as good as any. Why run three pipes from the mash house to all 12 fermentors when you can drop the small grain into the cooker and run one pipe.