Is "hararo" equally acceptable here? I thought "hararojn" would be more correct in this context - wouldn't "haro" refer to just a single hair, and "hararoj" to a group of hairs not necessarily a person's head of hair?
Apparently, haro means a single hair, like in many Latin languages. On the contrary, "hair" in english have no plural form (as far as I know), and means both a single hair (haro) and a group of hair (hararo), like, apparently, in many German languages.
See the Reta Vortaro to read the definitions and compare the multiple translations of haro/hararo.