Yes, it's heartening to see the rivals growing kind and protective as they mellow, isn't it? The bookend to Eliot's affection for Frost late in their lives is Frost's treatment of Ezra Pound. (Now there's a poet with a formidable biography! Keep it out of the bookcase--you don't want it to infect the "Cantos.") The flamboyant, 24-year-old expatriate "discovers" Frost during his English years, reviews his first book, introduces him around, and generally patronizes him. Eventually Frost gets fed up. He's 11 years older, competitive, and impatient with Pound's elitist obscurity. He's a better classicist, as Parini nicely emphasizes, but wears his erudition much more modestly. It's not long before the prickly pair fall out. They say mean things about each other in public and private (which Meyers repeats and Parini doesn't). Decades of hostility roll by. Then, in his 80s, Frost goes all out to get Pound released from St. Elizabeth's Hospital for the Criminally Insane, where he's been imprisoned for broadcasting nutty pro-Nazi propaganda during World War II. Of all the prominent American poets, conservative Frost has the strongest pull with the Eisenhower administration.
By the way, adding to your list of good things about Parini's book: I liked his discussions of Frost's politics. The old-fashioned populist loves nature and American self-reliance, like Teddy Roosevelt and Emerson. He's out of step with the artistic set during the '30s, when socialism comes into fashion--especially since his college gigs protect him financially from the worst of the Depression. Then, after the war, the country grows conservative around him, and he looks prophetic. But however strongly he feels about politics in the abstract, he doesn't let them interfere with his friendships.
I also liked how Parini showed Frost salting away poems to publish later. I can't seem to find it--can you?--but there's a lovely, insightful passage in which he talks about Frost's hoarding the poems so he'd always have some saved up in case he lost his touch. And like you, I thought Parini was very good on what I agree are the most important aspects of Frost's poetry: its breathtaking music and what you call its "spiky-Zen" quality--the way it snakes around, continually undercutting its own implications. (I think I can answer your parenthetical about why unmusical poets often have the best grasp of prosody. It's their very unmusicalness. That whole organ of musicianship is just going to waste in the brain. Gotta use it for something. Hmm. Any neuroscientists out there up for putting some poets in an MRI scanner and comparing their brains with Yo Yo Ma's?)
But I disagree with your comment that the indistinctness of the supporting players in Parini's biography was Frost's fault. They're not indistinct in Meyers' book. Of course, Parini might argue that Meyers gets them wrong, but at least Meyers leaves the reader with vivid pictures: chilly, overwrought Elinor; generous, playful Untermeyer. It's partly that he bothers to describe the characters when he introduces them. Here's Kay Morrison, Frost's secretary and muse after Elinor's death: "Frost's 'devoted, astringent, and affectionate amanuensis' had a slight build and bright auburn hair. Well dressed, elegant, and soignée, she often mumbled her words and squinted her eyes. Wade Van Dore said she was self-possessed in moments of crisis and that 'outward calmness seemed to be hers by birthright.' "
My favorite walk-on character was Dorothy Canfield Fisher, who helped Frost house-hunt in Vermont. She wrote sincere, lofty novels, such as The Deepening Stream, which haven't aged well (though fashions can change: Maybe the 22nd century will love her). Her children's book "Understood Betsy," a childhood favorite of mine that's still in print and devoured by 9-year-olds, reads like a parable out of Frost. Its young heroine, a sickly orphan, lives in the city with nervous aunts who project their terrors onto her by "understanding" what they take to be her timid nature. She blossoms into rosy-cheeked self-reliance when she's sent to live with cousins in the country who assume she's brave and competent. The insular, home-schooled Frost children turned out odd and not entirely socialized; it was interesting to think of them in relation to Betsy. (Of course, they were already mostly grown by the time Frost met Fisher.)
Soon, let's talk about some poems. I also long to know what you think would have happened to Frost's family and oeuvre if Prozac (lithium?) had been an option.