Equites sagittarii (late 2nd century)

[jho]

Basically it is not necessary anyway, probably every doctor in Medicine is able to quickly tell what happens to your neck when you drop of a horse backwards wearing one of these helmets.

*snap*

It nowadays is also possible to wear a cake on your head while sitting on a horse, but we wouldn´t expect the Romans to have done so.

I can agree that we cannot positively establish cavalry use based on the available evidence but EXCLUDING the possibility of cavalry use based on what you think a doctor would think of medical implications of wearing it does not appear to meet the scientific standards for verifying a hypothesis you have upheld in your own post.

there is a larger series of helmets that qualify as "niederbieber" but don´t have neckguards or just short ones. They are in HRR "Cavalry Sports I"

I assume that there is a typo in your post. These helmets (in continental Europe referred to as the Guisborough type) may qualify as "Cavalry" (?) helmets but they have nothing in common with Niederbieber helmets.