"Kaguya consumed the fruit of the God Tree and became the first wielder of chakra on Earth, before later merging with the tree to become the Ten-Tails."
Your facts are in the wrong place.
At bold, she didn't per say "fused" which is indeed a synonym to merging but, they're in-fact two different words. Not the same. She took on Ten-Tails within her to empower her even more. Let's back track, Ten-tails is the combined form of Kaguya and the God Tree. So it doesn't make sense to say "fused". Now about her will.
She had lost control once she had became the "Ten-Tails". The God-Tree overtook her mind so to say, and simply acted on it's on accord (will). The tailed beasts, I doubt have her will, if anything, the tailed beasts would have the will of the God Tree.