“Jamie dies first and keeps his promise that he’ll find Claire in death, and as he waits he takes advantage of opportunities to see her on the holidays when the spirit and real world are open to each other, like Samhain.
“If DG says that the series ends in Scotland, then I’m guessing Jamie dies in Scotland, and so his ghost can only roam there, so we don’t see his ghost visiting her in Boston, etc.
“Claire would only be in Scotland on these kinds of holidays at the beginning of book 1 and maybe once or twice more while she’s with Jamie in the 1740’s before/after Paris, so this time we see him visiting her is kind of his only opportunity.
“Maybe every year he visits her until she dies and joins him.”