Like you, I am intrigued by Falling Man's split in perspective between Keith and Lianne, the survivors of 9/11, and the terrorist identified only as Hammad, a follower of Mohamed Atta, whom the novel puts on the first plane to crash into the World Trade Center. (He doesn't seem to be based on a real figure: There is no one by that name among the 19 hijackers.) As a fictional character, Hammad doesn't strike me as particularly persuasive; DeLillo's account of his actions in the period leading up to 9/11 seems to be animated more by conventional wisdom about the behavior of terrorists (the anonymous apartment and lowly jobs, the studied inconspicuousness) than by a truly novelistic inhabiting of such a figure. That changes with Hammad's last scene—the epiphany you quote, which I too find remarkable, all the more so because to see the towers through Hammad's eyes requires entering his imagination (since he is not actually looking at them).
But are you sure that DeLillo draws such a firm distinction between the hijacker and the rest of his puppets—excuse me, characters? Because after that moment with Hammad on the airplane, the narrative cuts with shocking speed directly to Keith. Here is the relevant passage:
A bottle fell off the counter in the galley, on the other side of the aisle, and he [Hammad] watched it roll this way and that, a water bottle, empty, making an arc one way and rolling back the other, and he watched it spin more quickly and then skitter across the floor an instant before the aircraft struck the tower, heat, then fuel, then fire, and a blast wave passed through the structure that sent Keith Neudecker out of his chair and into a wall. He found himself walking into a wall. He didn't drop the telephone until he hit the wall.
We start the paragraph still looking through Hammad's eyes (and it's a brilliant touch that the last thoughts to pass through his mind are about the most prosaic of objects, rather than jihad or paradise, but we end it with Keith. So linked are they—and not just by physical proximity—that the pronoun does not even change. This is DeLillo's most explicit connection between the two men, but the similarity in their behavior is a leitmotif of Falling Man. Both discover blood on their clothes without knowing where it came from. Both seek solace in ritualistic acts—for Hammad, the physical preparations he makes for the attacks; for Keith, the arbitrary rules that govern his poker playing and the physical therapy exercises he compulsively repeats. Both distill their lives down to the essentials. Hammad "prays and sleeps, prays and eats"; Keith reduces his sleep to five hours to have more time at the poker table. "There are standard methods and routines," DeLillo tells us of the poker game, but he could just as easily be speaking of suicide terror. The outcomes are different, but the processes are the same.
The question, of course, is to what end? What purpose does it serve to correlate a survivor of 9/11 with a terrorist in this way? I find your argument about plotlessness very persuasive: Hammad, in the end, takes the easy way out by submitting to a religion that admits no room for doubt (in this interpretation, anyway), while Keith and Lianne lack the solace of a definitive end. DeLillo's figures have always been a bit helpless in the face of "the confusion of the real," as you put it; one of his signature mannerisms is to stop the narrative for an incantatory recitation of facts, often commercial trademarks (White Noise: "Orlon, Dacron, Lycra Spandex") or technical language (Hammad in Falling Man: "The windshield is birdproof. The aileron is a movable flap"). Lianne's Alzheimer's patients, too, futilely seek to impose a narrative on their lives, writing stories to slow the unraveling of their memories.
At the same time, though, I'm troubled by the aesthetic critique of terrorism that this reading of the novel implies. The depravity of terrorists, to state the obvious, is not that they seek to impose a tidy narrative on messy reality! In fairness, that takes things a big step further than you, or DeLillo, do. The towers may be "concepts" for him in an earlier novel, but to Keith they are no abstraction. And this is why the retelling of the event is so important to DeLillo: to emphasize the reality of it all, the physical brutality.
You mention the "pure ritual" of the poker table, and that's finally how I understand all the spiritual quests in this novel—as quests for purity. The "Falling Man" of DeLillo's title, as you pointed out, is both the man falling from the towers in the now-famous photograph and the fictional performance artist who follows in his wake. (In a moment of classic DeLillo-ian irony, a newspaper article about the artist is headlined "Falling Man as Heartless Exhibitionist or Brave New Chronicler of the Age of Terror?") But the phrase also carries intimations of the first fall of all—the exile from Eden. In the traditional Christian framework, we all live in a state of fallenness, cast out of a paradise that can never be regained.
In this novel, the attacks of 9/11 represent a second fall of sorts—for one need look no further than DeLillo's Cosmopolis to see that New York at the turn of the millennium was a prelapsarian gilded age. "By the time the second plane appears," Keith comments as he and Lianne watch the endlessly cycling video of the attacks, "we're all a little older and wiser." The novel's first pages chant a mantra of transformation—"This is the world now." The sense of exile, of loss, is palpable throughout the book. Lianne ruminates on a haiku that, fittingly, she remembers incompletely: "Even in Kyoto—I long for Kyoto." She identifies with it so thoroughly as to transpose it: "Even in New York—I long for New York." The New York she longs for exists only as a kind of purgatory of lost souls. ("The dead were everywhere, in the air, in the rubble, on rooftops nearby, in the breezes that carried from the river.") By the novel's end, Lianne seems to have attained a state of spiritual awareness that mimics the peace of her pre-9/11 existence: "She was ready to be alone, in reliable calm, she and the kid, the way they were before the planes appeared that day, silver crossing blue." (Interestingly, and disturbingly, Hammad is the only character who is spared this fall, who never enters the post-9/11 world. Does this mean he inhabits a form of paradise?)
I agree with you that the great post-9/11 novel, when it finally appears, will have to represent this entire fallen world. The best literature about any major historical event—slavery, Hiroshima, the Holocaust—depicts not only the trauma itself (which we can learn about from journalism or history) but its impact on individual human beings over time. Such a novel might usefully take an approach similar to W.G. Sebald's in his writings about the Holocaust, keeping 9/11 on the margins but allowing it gradually to seep into every aspect of the work, so that in the end it becomes the foundation for everything. On the front page of the New York Times this morning, a teaser headlined "Death Linked to 9/11 Dust" caught my eye: The article (buried deep inside the paper) reported that New York's chief medical examiner had attributed a woman's death, months after the attacks, to dust from the twin towers that she breathed as she fled the area. I couldn't help wondering if some future Don DeLillo, tomorrow or 50 years hence, will find here the catalyst to loose the confinements of history.