I only continue to question this interpretation:
It appears he has had some experience with the Willow before and has learned how to exert some control over it
Yes he can control Old Man Willow when he arrives on the scene, but I think he is master of a not-completely-obedient old grumpy tree. If Old Man Willow draws unsuspecting victims to him, I think this is of his own accord. Tom speaks to him as one would chastise an errant dog jumping up on a visitor, not as if OMW had fulfilled some order or plan of Mr. Bombadil. In other words... He can control the tree, but it is reactive, rather than premeditated, IMHO. He is the master, but that does not mean he controls, rather he oversees what happens and decides whether to do anything about it.