A gradient of gender neutralisation by number
Hypothesis examined: the further from the singular, the more gender fades: marked in the singular, partially neutralised in the dual, fully neutralised in the plural.
The first two degrees are right: the singular distinguishes هَٰذَا / هَٰذِهِۦ, ذَٰلِكَ / تِلْكَ; the dual pronoun هُمَا does not mark gender (rule “The dual pronoun does not mark gender”).
The third degree is wrong. The plural does not neutralise gender: it distinguishes هُمْ (masculine or mixed) and هُنَّ (feminine), and verbs have a feminine plural. The corpus gives clear examples: « هُنَّ لِبَاسٌۭ لَّكُمْ وَأَنتُمْ لِبَاسٌۭ لَّهُنَّ », "they are a garment for you, and you are a garment for them" (2:187).
The hypothesis also contradicts the dual rule (“The dual pronoun does not mark gender”), which shows precisely that the plural maintains the opposition. The "gradient" does not hold.
In the text
هُنَّ لِبَاسٌۭ لَّكُمْ وَأَنتُمْ لِبَاسٌۭ لَّهُنَّ