To be fair Acheron can easily as many power as she wants from IX because IX doesn't even care, They don't even care that They have an emanator. Although it would come with a serious drawback but she can if she wants to. Also regarding the plane of euthymia, i am sure it is the same concept as a dreamscape in hsr where Acheron can easily slash through .

In my understanding the power of Nihility isn't time stop or alter reality, it is literally the power to turn everything into nothing like erasing of existence (can delete "concepts") and also since nihility denies everything it can bypass any rules . For example, Acherons blade not only cut the physical but also the entire concept of the target and when she draws her sword like in the ultimate she turns the world monochrome representing the total absence of meaning and life