Mystery guy has very distinctive lips, right? Hard to miss. Definitely the same guy in all three images, though in the first one he might be a little older. And by the way, Mary Anne had no living brothers and her only known uncle was childless (no known aunts) so mystery guy is probably not a first cousin either. He could only be a distant cousin or friend.
To help confirm the time period, I decided to look up the photographer named on the print of the mystery guy when he was looking about the same age as he is in the tintype. I did this by using the directory records on Ancestry.com. Year by year, I looked up a photographer by the surname of Applegate in Philadelphia at the address Vine and 8th Streets and found James R Applegate worked there from 1871 to 1904 (he'd worked as a photographer prior to this but at a different address so I ruled that out).
I was lucky that the print of mystery guy had the photographer's details on it, not all do. I was also lucky to find him in a group shot with my ancestor so I could get a good idea of who he was associated with in my tree and when. Though I'm still investigating the individuals and photographers of the other images in the pile, many of them are of the same style and therefore probably around the same time period. And knowing that the Rorers and Fallows were something of socialites (and therefore had friends with the money to purchase multiple prints of a photo and distribute them to friends and family) and what appears to be enthusiasm for photographs, it seems likely that they received pictures of friends or family members, perhaps even swapping them with ones of themselves.
I will be posting more of the mystery photos in hopes that someone with a copy of the same picture, who knows who they are, will do a reverse image search like Tineye and find my blog and tell me who they are. My hopes aren't high but if I never put them out there for people to find, they'll never be found.