Beauty of hash tree is that hash(commit) = hash(parent commit) ∘ hash(changes). You could compute that by hand if you wanted. But you can't compute merge by hand since it involves basically rebasing commits to a different tree which is non-trivial and prone to introducing errors. If there is 3-way merge you add some changes on top of that.

I don't know what's FF merge, what's no-FF and what you do when your FF merge turns into no-FF.