I can 
Adam Mair married Janet Stewart. They are my 3 x gts. Janet's dad is Alexander Stewart (He's the Thornbush one). And then of course his father who was married to Jane Walker.
The second Alexander Stewart in your list is Janet's younger brother - son of Alexander (Thornbush)
The first, can't see him in my tree. It's possible he was Jane's nephew. I've not checked out her siblings kids yet
BINGO!!!
The Alexander who died in 1865 is, for a reason we probably will never know, buried in a pauper's grave in the cemetery BUT, his son Alexander is recorded in the gravedigger's day book thus -
"Alexander Stewart, miner, Alicia Bank Orchard, aged 62 years, widower.
Parents - Alexander Stewart and Jane Walker.
Interred in the old churchyard on 9th July 1877".
There is a Plot Book for the churchyard but it only begins about 1878, so just misses the above Alexander. However, there is a Stewart family plot in that book with three post-1878 deaths recorded, Adam Mair and the two other Alexander Stewarts.
So, we now have the correct Stewart family resting place AND, I have the transcript of a headstone which probably doesn't exist any more. (I will have a look when the weather clears).
This is the transcription -
"
Erected by Alexander Steurt and Jean Steurt in memory of their daughter Jean who died 17th March 1838 aged 1 year."
Is this your Alexander Stewart and Jean Walker? Or, an earlier generation?