I suppose he might have wanted to avoid bringing her to such a central place for the Don Quixote family. The factory can't be undefended.
Usopp is once again digging himself quite a grave with his hasty lies. But I guess the situation might not be so bad if the other folks appear there at the same time. I bet Law is wishing he had a plan B, though. Destroying the factory behind Flamingo's back won't bring him much joy if he winds up dead or captured himself and Caesar is returned to Flamingo. Especially with Law himself losing the position of a shichibukai while Flamingo never lost his.