The only bpt that is coming up on the IGI and Scotland's people is that of Isabella, which you have - 24 Feb 1807, Glencairn.
Yes, we have run into the same problems – impossible to find a definite date of birth for Alexander and Robert. We know the approximate year because of the family gravestone in Holywood Churchyard:
In Memory of James Gibson, who died at Gribton on the 17th of December 1832 aged 59 years Also Robert Gibson, son of the above, who died at Wanlockhead 24th January 1848 aged 30 years Also Mary McCall, spouse of the above, who died at Gribton Lodge 17th Jan 1863 aged 82 years Also Janet Grierson, wife of Alexander Gibson their son, mason in Dumfries, who died there 24th August 1868 aged 63 years Also Margaret Gibson their daughter, who died there 25th October 1868 aged 35 years Also Janet Gibson their daughter, who died there 18th July 1871 aged 29 years Also the said Alexander Gibson, who died there 29th Sept 1878 aged 73 years Erected by Mary McCall his spouse
What is interesting is that there is a Thomas Gibson and Ann Callan having children in Glencairn at the same time. One of which is Alexander Gibson, bpt 12 Feb 1805.
Yes, we believe that is just a coincidence. I have been in contact with members of that family. Their Alexander fits in nicely, but it is the wrong family! I believe this family had children Alexander (1805), James (1811), Thomas (1813), Janet (1814), Samuel (1815), William (1817)?
Do you have the names of Mary McCall's parents?
Yes, as you correctly write, we believe it is Robert McCall and Isabel McKonnen (or Isabella McConner).
You have been most helpful!!