Apparently the growing up into a different looking face than as a child or toddler was some sort of glitch, which I assume because there is an option in the nraas mod addressing it.
I don't think Mortimer looks like Cornelia. Agnes does have an odd face, as does Morty. Cornelia's is more generic. I hate the hairdo the game gives her when she grows old.
I'm not sure if 'Manage the Dead' was an option without nraas or not, or if the ghosts were more active in the vanilla game, but the gravestones in Mortimer's backyard are of his ancestors. I think there might be one or two in the town cemetery too. Whoever's picture is in Mortimer's, Cornelia's and Gunther's family trees, are a ghost available in the game fabric that can and will eventually come out, and be scrutinised for resemblances. Because they are there, that makes me think that family had to have been played. I've moved a family to another town before without and then again with their deceased's gravestone in their inventory, then placed in their backyard. When they moved without it, their dead relatives didn't show up in their family tree, when they took it with them they did and the ghosts eventually came out and had relationship icons in their interface such as green and orange little people icons for father or mother, grandfather etc. And only the one's who they brought, and I'm pretty sure they needed any generation in between too. I suppose developers and creators could make whatever family tie and ghost they wanted but it seems like it would be more work and mucking around than just playing them.
The mindset and work ways of the people who created and managed Sims 2, who then moved into working on Sims 3, I think would have been to play them and create semi-genuine lore. Today the practices and thinking would be to just create it, and that's apparent with how quickly things move along in comparison. And I think the game and its characters were somewhat more precious to them back then.