I'll be adding info to the stockpiles so you can tell how many spaces are "reserved" by items that haven't been hauled yet, hopefully that helps.
With the mushroom logs, when you harvest wild mushrooms you get some spawn to create mushroom logs out of, which need a log and get injected with mushroom spawn. After several harvests (4 or 5? Can't remember), the log is used up and deconstructs itself, but it refunds the mushroom spawn for you to be able to create another. Effectively, the limit to how many mushroom logs you can create is how many mushrooms you find in the wild.