فَيَكُونُ الْقَيْدُ الْأَخِيرُ بِمَنْزِلَةِ الْوَاقِعِ بَعْدَ «إِلَّا»
Thus the final qualifier functions like the element placed after illā.
إِنَّمَا ضَرَبَ زَيْدٌ عَمْرًا
Zayd struck ʿAmr in particular.
Under the ordering rule presented here, ʿAmr is the final complement and receives the restriction: the relevant alternatives are other possible objects.
Simply translating innamā as “only” is insufficient. One must identify which constituent is placed in the decisive position and what alternatives are being excluded.