Is there a mod that can make the PC fight with a lightsaber using one hand like Darth Nihilus?
Not without replacing pc with nihilus (replace or disguise).
Is there any mods that change or add to the PC's fighting animations?
No there areally aren't. With our model knowledge limitations there likely wont be any for some time. :( At least that is my limited understanding.
Is there a .2da where you can change the animations around? Like switch critical strike with power attack?
Is there a .2da where you can change the animations around? Like switch critical strike with power attack?
You can change what animations are tied to what action around in animations.2da, but that still requires the animation to be present in the model used by the creature performing the action. I.e. you can make Power Attack use the Critical Strike animation and vice versa, but you can't make them use Malak/Nihilus variant of those since those are separate animations in those model files.
The value in the name column is what should be switched if you want to change animations for actions.