Spoiler Questions - Who was Whom?

First and foremost you have to understand that much of the film, perhaps even most, takes place in Louis's mind as a fabrication of or as flashbacks. Second, I'm not completely sure which parts really happened and which are imagined.

Yes there is a second son that he abandoned in Tahiti, the auditions were held to find a son to impersonate his son(assuming that he hadn't seen him for years or maybe hadn't seen him at all, he wouldn't recognize him). The mother in Tahiti must not have wanted them to meet(again?).

I believe the heart came from his son living in France, as that's who we see on the coroner's slab at the end with the huge gash in his chest.

The man who's throat he cut was part of a group that were camped out in the woods near his house and, as he read on the computer, were plotting something that sounds shady. Apparently Louis is either very protective of his privacy or a little paranoid or he dreamed the entire event, I'm not sure which.

The body in the ice looked like it could have been his french son, but it also could have been the guy whose throat he cut, or it might have been him imagining his son or the guy he killed under the ice.

The woman who broke into his cabin and was seen smoking pot a couple of times, and was laying on the snow with her heart cut out, etc was just a product of his imagination.

The Korean part is sketchy for me, I believe the heart transplant may not have happened until then.

The dogs racing at the end I'm not sure about, but Claire Denis has said that many of the characters are completely imagined, including the girl who breaks into his house, and the woman who keeps following him(she is supposed to be the angel of death). The woman with the dogs at the end(she was the same woman who lived nearby with a bunch of dogs in an enclosure), she and her husband/boyfriend/whatever were real. So there it is.

Le Français est la langue du cinéma