According to the St Helier parish records William de Ste Croix was baptised 13/4/1772. The record has his parents as Jean de Ste Croix and Elizabeth Mersier (slightly different spelling).
His godparents were not his paternal grandparents, they were Guillaume Patriache Snr and Marie Le Geyt his wife. I can understand how someone might have thought that Patriache meant Grandfather but it was actually a surname in it's own right. Some records do give the relationships of the godparents to the child but it would have said 'grandpere' and certainly not Snr.
This is confirmed by looking at the baptisms of Jean and Elizabeth's other children,
11/12/1768 Jean son of Jean son of Jean and Elizabeth Mersier, godparents Jean Perrechon and Elizabeth Perrechon
14/3/1770 Aaron son of Jean son of Jean and EM, godparents Aaron de Ste Croix and Esther Neel, his wife
13/3/1774 Elie son of J de Ste C and EM, godparents Elie Canivet and Marguerite Le Gallais, his wife
1812/1776 Elizabeth daughter of J de Ste C and EM, godparents Jean Pipon and Elizabeth Pipon, his daughter.
So from the baptisms of Jean and Aaron the paternal grandfather was also called Jean.
I only had a brief look but could not find a marriage for Jean de Ste Croix and Elizabeth Mersier but did find one for William Patriache and Marie Le Geyt in St Helier on 1/3/1754.
As you have found there are a lot of de Ste Croix and Le Geyt (although they may no longer be relevant) families in Jersey, I did not see any Mersiers or Le Merciers but can have another look on my next visit to the Archive.
Jersey had strong links with Canada, especially the Gaspé, through the cod trade, try jerripedia, theislandwiki, for some information about this.