Are you sure that the first William Valentine was born in Fiji in 1815? That is very early for a European birth in Fiji.
He probably married Adi Liku Tabua about 1840, but after coming to Fiji from wherever he was born. William Valentine was definitely living in Levuka about 1844, when all the settlers there were driven out, along with their Fijian wives and children to Nawaido, Vanua Levu by Cakobau.
These settlers had first come to Levuka in the 1830s, and the missionaries arrived about 1835. When the American Exploring Expedition arrived in Ovalau in 1840, they found approximately 40 houses there. The narrative of the fleet mentions David Whippy and Paddy Connel and some missionaries, but no other Europeans.