Fasti Ecclesiae Scoticanae: Kinnellar (Not Kinnellan). John Mercer, born 1626, son of Lancelot M, merchant, London and Agnes Beans; educated at Marischal College, Aberdeen 1642-6. Joined the Protesters in 1651; adm before 21st Oct 1651; was authorised to get an assistant, 7th Oct 1675, because of bodily infirmities; left the parish 27th June 1676. and resided in Aberdeen; died 21st Sept 1676. He marr Lilias (dies 31st Oct 1713, aged 77), daugh of John Row, Principal of King's College, and had issue: John, born 18th Dec 1656, died Oct 1671; Agnes, born 2nd Feb 1656 (Marr 19th June 1677 William Lindsay of Culsh); Thomas of Todlaw and Smiddyburn, MA, born 28th Jan 1658; Alexander, born 29th Dec 1659, died young; Christian, born 10th June 1662 (marr (1) Robert Arbuthnott (2) Alexander Robertson, min of Longside); Isobel, born 2nd July 1664, died 11th May 1733; Anna, born 4th Feb 1666; Alexander, born 12th Aug 1668, died 2nd Dec 1690; Margaret, born 3rd Aug and died Sept 1672; a child born and died Jan 1676 [Mercer Pedigree]
The snag with Fasti in this case is that both this and the item on Rev William Mercer are sourced from a Mercer Pedigree, and there is no way of knowing how accurate this pedigree is, and therefore how accurate Fasti is.
None of the above baptisms is on Scotland's People.
Looking again at the New Jersey book, I see that William Jr is said to have been born in 1715, shortly after his father's ordination, and that his father, William Sr, was born in 1696. This, again, looks doubtful. It would require William Sr to have been ordained and married in about 1714, at the age of about 18; and Fasti says that he was ordained in 1720.