I've discussed this a bit over on the Chakotay/Seven thread in the Voyager section, so I won't belabor the point, but the setup in Human Error is enough for me, following on as it does from Unimatrix Zero and Seven's reminder that she once experienced love and romance.
It takes two to tango. Seven may have been interested in Chakotay since "Human Error," but they didn't begin having any actual mutual romantic interaction until "Endgame."
The ending of the relationship with Chakotay in the book literally comes out of nowhere, and is done for a very strange bit of reasoning. "We're not going to be on Voyager, so I'm dumping you." Huh? And as hard as Chakotay fights to keep her when she tries to break things off in Endgame, it struck me as out of character for him to give up so easily in Homecoming.
Going from "We're stranded on a ship together in hostile space and may not see home again in our lifetimes" to "We're home and can go back to our old lives" is a HUGE life change. It's not remotely unreasonable that people would reassess a lot of things in their lives.
Besides, I don't agree that Chakotay fought hard to keep her. All he did was urge her not to make a hasty decision and give up on pursuing a relationship because she was afraid of getting hurt. It's not that he was madly and devotedly in love with her, just that he was invested enough in their nascent relationship that he didn't want her to abandon it for no good reason.
That was one of many things I didn't care for in the book, and hardly the biggest failing of the plot. But it was very much a "clearing of the decks" type plot moment, where the plot drove the action of the character rather than the other way around.
Everything you say about the end of the C/7 relationship in Homecoming describes what I think about its beginning in "Endgame." Having Chakotay reciprocate Seven's interest was a blatant case of the plot driving the character. It came out of nowhere and was arbitrarily tacked on to give Chakotay, who had ceased to have any personality whatsoever once Jeri Taylor left the show, something to do in the finale. I couldn't believe in their relationship, so I was glad to see it brought to an end so quickly and efficiently.
Full Circle, on the other hand, finally gives us the Janeway/Chakotay romantic subplot that the show would never quite commit to, and it's a pity that Janeway's death prevents further exploration of the idea.
I agree with the first part. That relationship was one that had an effective groundwork laid for it in the show, so it was a natural outgrowth of the established characterizations. However, I submit that Janeway's death did not prevent further exploration of the idea, because Chakotay's arc in much of FC is all about how he reacts to losing her. That is definitely an exploration of the idea of their relationship. The end of a loved one's life is not the end of your feelings for them or the effect they have on you.