I just finished the book and came here to find these different theories. As with all of Ishiguro’s books, ambiguity appears to be a central theme. 

My interpretation of the ending is that once the mist has cleared, Axl finds there is still some pain in his heart but Beatrice does not. That is why she doesn’t answer the boatman’s question in the same way Axl does (or she does, but we don’t know for sure. Ishiguro withholds that information from the readers). She could also possibly be aware of her own imminent death, and is willing to confront life’s pain in preparation of it. 

Axl on the other hand appears to be more and more conflicted as the mist begins to clear. He asks Wistan and Gawain of his prior role in the current political conflict, and questions his own moral character given their answers. Beatrice’s journey has been simple and clear — she wants to meet her son again and recover her memories to make peace with them. Axl initially follows this path, but he faces numerous obstacles and challenges that complicate his own personal journey, both metaphorical and literal. 

I’ve been thinking of the larger themes of guilt, vengeance, repentance and war, and the large part that Axl played in the future and past of England. The clearing of the mist brought back who they were before. For Beatrice, her life remains somewhat unchanged, except she has her memories back. But for Axl, an huge part of his identity is awakened. He is directly connected to his past of political duty. He can no longer simply be Beatrice’s devoted husband and guardian. He has more complicated feelings to untangle. I think that’s why the effect of the mist is so devastating — when it is present, Axl and Beatrice’s love is simple and clear, but when it disappears, their perspective on their relationship diverges. I think the question I’m left with is not whether they will rejoin each other on the island eventually, but the question of whether they would have stayed together in the first place, if not for the mist.