That is definitely the marriage; I have a copy of the record from SP. If that is from Ancestry, can you tell me if you see anything interesting there? Maybe an age? All I see when I click on Margaret’s name is an Ancestry ad.
The name Donald does sound familiar; probably something I found in 2016 that did not pan out.
If Alexander was in the army I would hope to find him in a later census - 1841, 1851, etc, and that has been my challenge. My suspicion is the family was in another county, not Ross and Cromarty. But if, with no birth year, it is that difficult to track them in one county, I don’t see finding them elsewhere - again, very common names.
But thank you for trying.