The date of Aragost's birth appears only in The History of Middle-earth volume 12, The Peoples of Middle-earth. It cannot therefore be considered completely reliable.
Many of the names of the later line of Isildur are difficult to interpret, but usually this is because they contain unfamiliar elements other than the common Ar(a)-, indicating their claim to royalty. In Aragost's case, we do have an attested Sindarin element gost, which means 'dread' or 'terror', but that seems strange attached to the name of one of the noble Northern Dúnedain. If this is actually the intended meaning, perhaps it simply implies that Aragost caused terrible fear in his enemies.
Given the close ties between the Dúnedain and Rivendell, it is not inconceivable that Aragost played some part in the rescue of Celebrían, but no records exist of the involvement of the Dúnedain.