It's not by random chance that of all the Coneely men, Sean Michael was the one that the seal saved, and it's not just because he fought hardest to stay alive either. How does Grandfather describe him at the beginning of the story? "Smart boy, dark hair, bit of a rebel." Sean Michael was a Dark One, and that's why the seal came to his rescue. Which also makes it a clever little bit of Foreshadowing, an early indication that the Dark Ones have a special connection with seals and that the seals look out for them.
Maybe with a little bit of Fridge Horror and Tear Jerker. If Jamie is about five when he rejoins the family, he will have missed out on a lot of the early social development that children need. This is especially important for learning language, where if a child doesn't learn to speak by a certain age then they usually don't learn to speak at all. He'll probably be mute all his life at best, or a feral Wild Child at worst.
Unless you go by Tadhg's idea that he's "just living with another branch of the family". A central part of the story is that Jamie isn't just a human child, he's a Half-Human Hybrid, and one with especially strong ties to his non-human side at that, so him spending his formative years with that same non-human species (the seals) wouldn't necessarily be the same as an ordinary human living those years with only animals for companionship. He'll still have to re-learn how to speak English and live as a human, but that's the kind of thing that little kids can pick up on relatively quickly (and seemingly he retains some relevant memories already, given that he was still practicing some human behaviors like laying out dishes on tables) — it's more like a radical culture change than coming in from isolation.
When Fiona ends up on Roan Inish by herself, Grandmother never finds out what really happened, believing afterwards that Grandfather took Fiona out with him that day. But how did Grandfather and Eamon figure out that they needed to go looking for her without even hinting to Grandmother that they didn't know where she was? Surely if Grandmother had actually seen them come into the house without Fiona, she never would have bought the story that Fiona had been with them the whole time, and if they didn't go into the house (where she was supposed to be), how did they realize she was missing?
