Hello everyone and thank you again for your help.
I have gone through the many sources that you each communicated. The one that, with the information I have, has given the most results for the moment, are Largs Kirk Session, which mentions a William Duff.
Largs kirk session, Minutes (1814-1854), CH2/923/1 (NB : Presbytery of Irvine <then in 1834> of Greenock / and Synod of Glasgow & Ayr)
Image number 1
July 31, 1814 - The Session being met & constituted […] admitted as Members of Session […] William Duff in Town […] they being this day ? ordained Elders of this Parish. The above persons […] now are all the present Elders of this Parish.
July 11, 1814 - The Session being met & constituted […] appointed William Duff on of their member to the Session Clerk & Treasurer […].
But an ordination in 1814 ? That made a William Duff born in 1792 maybe too young to be a ruling elder (22 years)... I found the answer, there is a second ordination of a William Duff in 1837.
Image number 31
December 3, 1837 - Which day the Kirk-Session of Largs being met & constituted, ? the Minister & elders, the Session considering that they had formerly resolved to add to their member the following Sessions, by ordaining them to the Eldership in this parish; viz: […] William Duff, Feuar in Largs, […] there all residing in the parish […].
As elders were ordained for life, it mentions two different William Duff, which seems confirmed with a second entry in the Ayrshireroots wibe site : 1831, Aug 30, William Duff feuer Main Street, Elder Treasurer, Parish Church (age 73).
So, Largs Kirk Session have already advanced my research, the father of William Duff (a1792-1842) could be William Duff (a1758-1831).
To be continued