Something that is worth considering is David Jackson's service record. I know it has been mentioned before by Ken, but I sent for my father's service record and found that he didn't just sign up for the war effort, but he was a regular soldier and had in fact served 7 years in the army. A lot can change in the real world during this time and if David was the same then his mother may have died before he left the army. If he befriended other soldiers in the army they may have kept in touch - I believe there is a specific site for this.
Secondly, it seems to me that David and his brothers were very close, it might be better looking for his brothers John, William and Robert in the hope that some of their of-springs can throw some light on the situation. I think that his brother William might have married Barbara Stirling Murray in Carluke 1929. Perhaps this is a line to pursue - but again the certificate would have to be purchased.
John