«جَاءَ الطُّلَّابُ غَيْرَ عَمْرٍو»
ghayra: accusative exception, first term of iḍāfa.
ʿAmrin: genitive second term.
In «مَا جَاءَ الطُّلَّابُ غَيْرُ عَمْرٍو», ghayru can be a nominative badal, but ʿAmrin remains genitive.
Always parse ghayr and its complement separately.