وَتَظُنُّ سَلْمَى أَنَّنِي أَبْغِي بِهَا بَدَلًا، أَرَاهَا فِي الضَّلَالِ تَهِيمُ
Salma thinks I seek another in her place; I consider her lost in this mistaken belief.
Wa-tazunnu Salma reports Salma's thought, whereas araha states the speaker's own assessment of her. Careless coordination could make araha seem part of what Salma thinks, which is not intended.
Fasl prevents that attribution. The sentences are thematically related, but joining them could distort who holds which view.