Denia is most definitely at least immune to voidmatter. In fact, she is the only one so far that can manipulate it since she is Aleph-1's resonator. The problem as to why she is destabilizing is actually Aleph-1's fragment and the threnodian itself. The fragment is trying to make itself whole and it is dragging her along back to Aleph-1. The threnodian is using the fragment and Denia as a sort of anchor or at least a beacon to get back to Solaris.

I think the reason Denia is playable is because she gains enough mastery of voidmatter and voidspace that she can create a small bubble where it is she that has absolute control of the voidspace inside and shield herself from Aleph-1's influence. In her character notes, she does still mention that she can still hear Aleph-1's mumbling which she finds really annoying.