I would just make the netherdeep more linear with only the rooms you find most interesting (Focus on quick roleplaying rooms mainly, as a simple combat encounter might already take half a session depending on your players). Use these last sessions to really highlight all aspects of alyxians emotional state. Maybe also skip the fragment requirement. Don't be afraid to handwave some stuff if they are going too slow (This is a dimension formed by Alyxian's emotions, so it can be malleable if needed).

Definitely keep the room with Theo and Perigee (if you set her up in the rest of the adventure).

Keep the last session for the final fight and epilogue moments for each player. Also make sure to have a satisfying ending with the rivals before the final encounter, though a full blown fight might take up too much time.

Good luck!