You seem to have missed Thomas, b 26 April 1863, and Jeannie, born 7 November 1865. Both died in New Kilpatrick in 1866. One or both of their death certificates will tell you whether their father died before them, which should narrow down the search for his death.
If Elizabeth was aged 2 on census day 1871, she would have been born in 1868 or 1869. Have you got her birth certificate? Her date of birth will further narrow the possible date of Alexander Scott's death.
Unless of course Alexander had left the family? Maybe he went on ahead to America to get settled in a job before sending for his family? Or maybe he just disappeared and Elizabeth made out that she was a widow to save face?
I see that Alexander and his baby son were staying with the Langs, who must be Elizabeth's parents, in 1851. (It's Blairdardie, by the way, not Blairsdale - are you using Ancestry's notoriously inaccurate transcriptions? If so you should always check by looking at the original document. See
https://maps.nls.uk/geo/explore/#zoom=16&lat=55.90208&lon=-4.36130&layers=6&b=1) Not surprising as they were married less than a week later.
Without a death certificate to tell you the names of Alexander's parents you are going to struggle to trace him further back.
There's only one baptism of an Alexander Scott in Renfrewshire in 1829-1832, in Neilston. He is in the 1841 and 1851 census with his family so he can be eliminated from your search.
This means that the record of his baptism, if it ever existed, has not survived. That may be because his parents didn't bother, or it might be because he was illegitimate.
You could read through the minutes of the Paisley Kirk Sessions, which are available at
www.scotlandspeople.gov.uk, to see if you can find any mention of either a man or a woman named Scott being summoned before the Kirk Session for an unmarried pregnancy. Also to see if there is any mention of the parish looking after Alexander when he was too young to fend for himself.
Either way it looks unlikely that he has anything to do with the Scott family mentioned earlier in this thread, because the head of that family was born in Ireland after Elizabeth Lang described herself as a widow.
In order to avoid confusion, you could consider asking a Moderator to split your query off into a separate thread of its own.
