They determined early on that Harriet was looking across the street at someone else during the Children's Parade, and they did the detective work to find a photo of that reverse angle. That was fine — clever, even. But once they had that photo, they never thought to show it to Henrik and ask him if he recognized the man in the blue sweater? Henrik told them at the beginning that he believed the murderer was someone else in the family, and he should have been able to recognize his nephew Martin — Martin, who had lived near Henrik for his entire life and had succeeded Henrik as the leader of the family/company. Henrik would have recognized Martin even in a blurry, decades-old photo like that.
I know Henrik was supposedly incapacitated by a heart attack for the middle part of the movie, but a heart attack is not a stroke or a coma. He was awake and aware and back at home within a week or two. Just a week or two in the middle of an investigation that took several months. Seasons changed, for crying out loud. And they never showed him that photo?
No, I don't buy it.