I have found online a history of St. Andrew's Church in Williamstown. In it my first question was answered. James Roy GRANT was, indeed, an Elder of the Church. He was ordained in 1833.
I have also found the answer to the second question in the records of baptisms at St. Andrew's. James Grant was the son of Alexander Grant and Ann McDonnell.
I've decided to leave this query here even though I've answered it myself in case another relative should come upon it in the future with the same questions.