Hello I cannot help you with the Duthie connection but can provide information on the Crombie connection. My mother was a Crombie, descended from Alexander Crombie (1760-1840) who was the cousin of your Alexander Crombie and inherited Thornton Castle (Marykirk, Kincardineshire) from him when he died childless in 1832. You probably know that your Alexander Crombie was a wealthy Advocate and Land Agent in Aberdeen. However, I have not been able to find out much about his antecedents (or those of my Alexander). From what I have discovered (from his will) there seemed to be a lot of people with Crombie as a middle name in Aberdeen at this time - possibly because he was their god father or their fathers were business associates.
If you know anything about the origins of these Crombies I would be grateful for any information. If you want to know anything about the Crombie descendants of my Alexander I could provide you with information.