The navmesh here was EXTREMELY bad. Huge areas where navmesh was going under walls and into nonexistent rooms, flying navmesh, random gaps, it seems like the room was significantly redesigned after navmeshing was done. Also placed a rock to cover a gap I noticed.
Tested and my followers and enemies all pathed correctly, Serana was still in the sarcophagus when she first appeared, all seemed working. The navmesh still gets a bit odd around the pillars when the sarcophagus rises out of the ground because the navmesh rises with it and isn't quite aligned so it floats, but I didn't want to mess with that too much, Serana was able to follow me out.

Attached Files:
dimhollow.esp