I've just had another trawl through the 1841 census sheet showing James Maitland living in the household of James Buchanan. It's actually from Old Kilpatrick or West Kilpatrick which are a good bit to the west of Milngavie, towards the River Clyde around the Bowling area. I think the top entry on the sheet, the address is "Hardgate". If so, that's only about 5 miles going by the back road from Milngavie towards the direction of Clydebank, or the main road to Dumbarton. It makes a lot of sense that this could be either Jean's father, or alternatively, a brother. Funnily, Hardgate is only a few miles from where I grew up, and I regularly walked up that direction, following the course of the old Roman "Antonine Wall".
James Maitland was born 1835, Jas. Buchanan 1797, so Jean could feasibly be born c. 1815, therefore, Jas. Buchanan could be from the same, or 1 generation back.