I think this may indeed have been a boring Book Club had you, my interlocutor, not turned out to be ... totally crazy.
If I read you aright, you're suggesting that in embracing sex tourism as a cure Houellebecq is in fact critiquing it as a symptom. This is too intelligent by half. Houellebecq seems to me entirely in earnest, or at least as much in earnest as in the post-human epilogue to TheElementary Particles. His satire, as you say, is slippery, so much so that it's not really satire—his plan for rationalized, legitimized sex tourism is not a sort of Swiftian Immodest Proposal (advertising tagline: "Very immodest") meant to jar us into conscience by its very extremity. He genuinely appears to have decided that the solution to Western sexual unhappiness is to outsource the sex; it's after all not the obsession but the competition, sex as a status marker that he finds abhorrent. And really some of his theories in this book—about the inability, for example, of Western women to "love"—are so stupid and unoriginal that I can't imagine him writing them for any other reason than that he believes them.
You are impressed by the integrity of the tone; the tone strikes me merely as his customary one, tone by rote. He returns to the first person of Whatever and manages to sound, despite the intervening years and a new translator, exactly the same. In that book he achieved a distinctive sort of affectless profanity: "After Tisserand's departure I slept fitfully; doubtless I masturbated. On awakening my tackle was sticky, the sand damp and cold; frankly I'd had enough. I was sorry Tisserand hadn't killed the black guy; day was breaking." The narrator seems always to be waking up, observing a few things in a detached, unhappy manner, and going back to sleep. Maybe a third of his comments will offend us; he doesn't care one way or the other; he notices, his eyelids rise long enough for him to notice, and then that's that.
This time around he seems intent on offending—it has turned out to be where the money lay hidden—but, like his characters who don't ever know how to begin to behave, he's not sure just what offensive is. I get the sense that Houellebecq was surprised by the furor surrounding The Elementary Particles, and now he's like a man who made everyone laugh with some accidental facial expression, and he's trying to make the face again. So in Platform the narrator tells us that at Omaha Beach "a bunch of morons died for the sake of democracy"; that upon seeing one of his fellow tourists in Thailand he wants to "put [his] fist through her fucking face"; that he doesn't like children because they scream and defecate a lot. It's all fairly unpleasant, maybe, but it fails to provoke, and though I won't say that it's calculated it does seem warmed-over. Compare the line about children to Bruno's reasons in Elementary Particles for withholding his right-wing essays from publication: "I had to stick to my 'liberal-humanist' position; I knew in my heart it was my only chance of getting laid."
Platform is also the less despairing book. There is his embarrassing sentimentalization of Valérie, who not only is magnificently endowed, breasts-wise, and shy, submissive, eager to please but also—rich and ambitious! She likes threesomes and is expert at instigating them. Meanwhile Michel, the narrator, is always performing complex tantric sex maneuvers to keep himself in the game. Is this our Houellebecq, friend to the sexual pauper, defender of the pervert, look-out for the masturbator? There are parallel scenes: in TheElementary Particles, Bruno responds to rejection on the beach by lying down on a towel next to a group of teenagers and secretly pleasuring himself. "It was done," the scene concludes brutally, and the humiliation is intense. In Platform, same beach, same rejection, but Michel's response is to approach the towels of the girls tormenting him and—steal their copy of Elle magazine. In TheElementary Particles, Bruno was forced to read up on sex and do exercises; in Platform, Michel already knows the tricks. And Valérie's large breasts put me in mind of a little problem Bruno discovered in TheElementary Particles once he finally achieved his dream of a swinging, sex-club existence—"Christiane told him time and again it didn't matter… but [he] couldn't help feeling that many of the women they met in clubs were somewhat disappointed. … No one ever commented; their courtesy was exemplary, and the atmosphere always friendly and polite, but their looks couldn't lie and slowly he realized that from a sexual viewpoint, too, he just didn't make the grade." There is nothing even remotely approaching this sort of toughness, honesty, and (because they come to the same thing) sympathy in Platform. It's amazing what celebrity will do for the sexual prowess of one's fictional alter ego.
I think you're absolutely right that between TheElementary Particles and Platform this book is tonally the more like Céline. But what would a Céline look like, now? He'd be a post-something Céline, as Houellebecq is, and he'd have to play a very complicated game to avoid seeming to practice what Orwell called "a despair that is at least partly a pretence" when he compared Evelyn Waugh to Céline in 1940. A while ago a French interviewer, trying to pin Houellebecq down politically, asked if he sympathized with de Gaulle, to which Houellebecq replied that no, if he'd been around during the war he'd have been a collaborationist. I mean, in a way, what else could he have said, what with four-fifths of the French intelligentsia claiming they were active in the Resistance? But Céline really did collaborate, he really did call for the murder of the Jews, he was guilty. There might be something of this venom in Houellebecq's feelings toward Muslims, though more on that next time, and also possibly toward women ("all those inducements to multiple rape," Céline called them). But let me for the moment define Céline as savagery plus pity. TheElementary Particles was the first book in a long time to possess both those qualities to excess; Platform only gestures in their direction.