I am still worried that almost all the recorded Cathel Kerrs were from Assynt in Sutherland, but your Castle was said to be from Argyll. I can't imagine anyone confusing Sutherland with Argyll, because Ross and Cromarty and Inverness-shire lie between them.(The Argyll and Sutherland Highlanders regiment of the army was founded much later, in 1881, so that doesn't come into it.)
A long shot re Hannah, wife of Alexander, maiden name unknown. There is one digitalized Nova Scotia newspaper on line that might have listed the marriage - see Acadian Recorder. https://lamns.ca/initiatives/nova-scotia-historical-newspapers-online/
I am still worried that almost all the recorded Cathel Kerrs were from Assynt in Sutherland, but your Castle was said to be from Argyll.
Yes, sometimes children can be quite old by the time they were baptised. I have one family in Scotland who baptised 5 children on the same day. I think because there had been a new church of their denomination built near to them, whereas before they would have had to go a long way to get to a church.It's not uncommon to see two or three children baptised together - there could be a variety of reasons for this. Perhaps their parents were too busy to find the time, or unable to find the fee, or were sick, or away from home, or as above, or the church was just too far away to get to. Many reasons.
On this note, here's what I just read about Assynt.
At 1832 when the Will was written what do you know about the older children named....eg...location, marriages etc -"my Daughter Margret, my Daughter Catherine, my Son Castle and my Son John"