During the Middle Ages, when Passion and Mystery Plays were performed by guilds for whole towns, The York Mystery Cycle presented Joseph's dilemma as a combination of both Suspicion theory and the Humility theory. His lines before the appearance of the Angel, as I recall, go something like this:
"Oh God, I'm sorry I've been so foolish. Why would I think that a pretty young thing like Mary would be interested in an old coot like me? My hair is gray and my beard is scraggly. I wouldn't have the energy to be the kind of husband a young woman wants these days. I'll just quietly send her away, and do my best to provide for her so that she and the Baby don't come to any harm."
So he is suspicious of Mary's fidelity, but humbled by his recognition of his own inadequacy. Rather than punishing her for what this play presents as Joseph's doubt's, he decides to send her away.
I wish I could do justice to how funny Joseph's lines are. Every man in the audience would have howling at Joseph's lament.
So maybe the answer is not suspicion or humility, but both as part of the human condition. It certainly makes for a deeper performance by actor's, and is a more human response.