I went back and read all the parts I'd originally skipped over, and strung them together with the cannibalism parts to make the whole story.
The figure who lingers with me is not Sarah Graves (the widow referenced in the book's subtitle), but Tamzene Donner. (Apparently that's how she spelled her name, though I've always seen it spelled in other accounts as Tamsin or Tamsen.)
Would anyone ever write her as the heroine of a love story? Her husband was some twenty years her senior, and three times widowed; she had already lost a husband and children herself. It's hard to imagine their marriage was anything but the union of two clear-eyed souls settling for what they could get at this point in life.
And yet. Through love, or maybe duty, or maybe just panicked poor thinking, she stayed with him until the end. She had four chances to leave their camp and go over the pass: the "Forlorn Hope" expedition for help (fifteen of the youngest and strongest party members, including Sarah Graves, set out on foot to send back rescuers and also to remove themselves as competition for the scant food resources), and three separate relief parties who were dispatched once the Forlorn Hope survivors reached civilization.
Tamzene Donner stayed behind every time. She sent her children to safety (or so she hoped) with the last two relief crews, but wouldn't leave her husband, who was dying of an infection in his injured arm as well as the starvation and cold that plagued everyone else.
It's hard to know, from this distance, what her thought process was. The last relief party described her as "distracted;" she asked them to please give her time to check on her husband (whose shelter was at some distance from the cabin where her children were) before she decided whether to come with them, so plain mental paralysis might have accounted for her ultimately declining to leave.
But it's also possible to imagine it this way: she knew her husband had no chance of making the journey down the mountain. She knew he was going to die, and she made up her mind she wasn't going to leave him to die alone in that terrible, terrible place, even though the longer she stayed, the greater the chance she'd die herself.
She did die. When a final relief expedition arrived at the camp, they found only one survivor, who confessed to having eaten her remains. (There was rampant speculation that he'd actually killed her for the purpose.) She must have known, staying behind, that there was a good chance she'd never see her children again, but stay she did.
Is this only a noble act if she was motivated by romantic love? Is it, perhaps, more noble if she wasn't in love with her husband, but acted out of Christian charity, or wifely duty, or simple empathy for a suffering man? It's hard to decide. One of the reasons I'm drawn to historical romance, though, is that I'm fascinated by all the reasons besides love that could lead two people into marriage.