I believe the Deacon title was because Alexander was a Deacon in the Church of Scotland. The position is very rare nowadays, but there are a few Deaconesses. They were semi-trained for ministry.
I have to disagree with this.
According to the Church of Scotland's own web site, deacons have been ministering in the Church of Scotland for 120 years. In other words, only since the late 19th century. See
https://www.churchofscotland.org.uk/serve/ministries-council/partnership-development/the-diaconate. It also says there are currently 58 deacons. It coyly avoids including what Wikipedia says:
The permanent diaconate was formerly exclusively female, and it was in 1988, the centenary year of the diaconate, that men were admitted to the office of deacon..
As far as I know the Church of Scotland has not had 'minor ecclesiastical orders' since the late 17th century, and possibly earlier. You were either an ordained minister, or you were not. No vicars, rectors, curates etc (and no bishops, canons or deans either, of course). I've never come across the term 'holy orders' in relation to a Church of Scotland minister, only in relation to an Anglican/Church of England clergyman, though it might also have applied in the Episcopal Church of Scotland.
In the 18th century most towns had guilds of artisans and merchants, and the senior officer of a guild, corresponding to chairman or president, was the Deacon. Think of Deacon Brodie, the 18th century model for 'Dr Jekyll and Mr Hyde'.
The following is just one of numerous examples from the records of the Forfar Incorporation of Shoemakers, who elected a deacon, thesaurer (treasurer) and officer (?secretary) every year:
Friday 27th September 1665. The trade being conveened in order to choose their deacon and there being on leit with the present deacon viz James Binney elder John Auld John Whytlaw David Ramsay and David Binny the said James Binney was continued deacon George Binny thesaurer and Alexander Dickson son to John officer till Michaelmas 1666. Kilsyth was a burgh of barony, with its own town council, so it probably had a guild or two. A small tenant farmer might also have practised a trade alongside farming, and could have been a member of a trade guild in the town.
If I were you I would look for the records of Kilsyth Town Council to see if Alexander Rennie is mentioned in those.
