If I understand the original enquiry correctly, what you are asking is whether the combination of the two witnesses at James and Elizabeth's marriage being Thomas and David Ward and the birth of three brothers James, David and Thomas being born to the same parents (along with the 4 other children) is sufficient evidence to assume that the James born to David Ward and Jane Robinson is the James who later married Elizabeth Parsabell.
The answer, I would suggest is "maybe". What you have is strong circumstantial evidence, but not definitive proof. On the other hand, for records before statutory registration, it may well be all the evidence you are likely to get. Whether you consider it as sufficient to add to your tree, is, ultimately, down to personal choice. Do YOU think it is strong enough evidence?
For myself, I would probably consider what you have as strong enough to add to my tree, knowing that it might later prove mistaken, but I would be researching the other brothers, looking for any common links or further hints.
I had one instance of a Janet Howie in my tree who emigrated to New Zealand. The evidence linking her to my tree was tenuous, but I had a feeling it fitted, so I added her in and followed her descendants. Eventually I found a marriage reported in the New Zealand papers of a descendant of hers who was described as descended from the "lairds of Tarbolton and Riccarton" (a slight exaggeration, they were tenant farmers) which confirmed the link. Later still I found that Janet's father lived to 1856, and the statutory register of his death proved that he was indeed a son of "my" line.
