Was the "Y" actually a "thorn" or not" ?

The Y in Ye Olde Frappe Shoppe was a Þ. The one in you was not. The OE dative and accusative 2nd person plural was Eow. Incidentally, the subjective and objective in the 2nd person plural were ye and you respectively. In the 2nd person singular they were thou and thee with the vowels reversed.