Phoodit is often after Nyda, partly because Saiparn always tells him to go aways once he tries to get close to her and only Phoodit can calm Nyda down. No one else can.
You are totally right. Saiparn has started to see her previous self through Nida's twisted behaviours. She can relate to Nida and blame herself for this.
Maybe the writer intentionally ignored Saiparn in the storyline to provoke more sympathy towards her. Nida...too much of a psycho episode. I cant imagine someone who is this crazy in love. First love during that period might have been this serious. Sigh....