I agree that Armand would be a bigger part of the story. I could see him either becoming more bitter and, eventually, dying alone (or committing suicide) or he would seek out Desiree to make amends. Either way, Desiree made the right decision and her life would certainly be better than Armand's.
I think we are forgetting the impact that the letter of his mother would have on Armand. Perhaps to think of a more optimistic outcome, we could surmise that Armand would realise the error of his ways, be overwhelmed with guilt and seek out his wife and child and make amends with them. Or, to be more pessimistic, he might be overwhelmed by the contradictions within his own racial heritage and kill himself, unable to accept that it is actually he that does not have a pure bloodline.
Excellent question! Armand would almost certainly become more and more bitter. He might even find some pretext for divorcing Desiree or arranging for her to move elsewhere. Or he might leave the area himself -- perhaps to go back to France. In any case, it is hard to imagine him standing by and watching their child grow to adulthood.
You could get a lot of answers to this question, but my assumption is that Desiree and the baby would leave the area and perhaps move to the North where a mixed race baby would be more accepted by society. It doesn't seem likely that her husband would have a change of heart and accept her, the baby, or his own heritage anytime soon.