The first time we see adult Elijah, he's talking about a comic book artist drawing villains with disproportionately-large heads. The way the scene is shot, we only see his reflection in the picture he's describing. Elijah's hair makes his head look oversized.
In the same scene, he also says superheroes often have a square jaw. In this film, the hero is played by Bruce Willis, and near the the end of the film, the artist's rendition in the newspaper (based, presumably, on descriptions given by the children he rescued) gives him a very impressive Lantern Jaw of Justice.
Elijah's mother talks an art piece in Elijah's gallery, noting that the enlarged villain's eyes indicated a distorted viewpoint. Bruce Willis is a famously squinty guy, which makes Samuel L. Jackson, who has one of the most intense gazes in Hollywood, seem like his eyes are bigger by contrast.
"I see guys like you all the time in my work." David is a security guard with a talent for spotting troublemakers. He identified Elijah as the villain in their VERY FIRST MEETING!
During the conversation where Elijah asks David about his ability to detect evil-doers by sheer intuition, only in retrospect do we realize David has seated Elijah in the yellow section, which is one of the colours that shows up around evil-doers.
David's Kryptonite Factor is drowning because the increased density of his bones makes him less buoyant than other people, and thus he tends to sink and drown easier.
The fact that "Mr. Glass" is exactly the sort of name a comic book supervillain would have may not be apparent to some viewers, even after the Twist Ending.
The first time I saw the movie, I, like David, assumed that his powers didn't always work after M. Night's cameo as a drug dealer who manages to give off a false positive for David's "Crime Vision." However, after a recent second watch, I noticed the bright colors of people's clothes for the criminals. The drug dealer was wearing a red and blue jacket when he grabbed the package from the trash can. When David confronts him, he's wearing a solid blue jacket. He switched clothes.
Or David's vision was of another day.
Despite being unbreakable, David apparently did get knocked unconscious by the train crash. This seems like it might contradict his being invulnerable to injury ... except that direct impact (e.g. a blow to the head) isn't necessary for a high-speed collision to render a person unconscious, as merely having one's brain shaken hard enough to temporarily scramble the signals between its neurons can have that effect. Unbreakable, yes; unshakable, no.
Something like a train crash will throw out an absurd amount of dust and smoke. Since David still needs to breathe...
There are a lot of rivers and small lakes on the route that train was traveling. If the car David was in wound up in the water, he could have resisted injury from the initial impact but still passed out when it sank.
Even if David had been shot, and it just bounced off, his family would be in danger of being hit by the ricochet.
Given the train wreck did enough to knock him out, it's possible that David could have died from the gunshot hitting, for instance, his eye or throat. David's tough, but the implication is largely that his bones and muscles are strong, which wouldn't help in some vital spots.
Mr. Glass casually turning down a high-paying customer because his motives weren't perfectly in tune with his own. It seems insane, and it's inexplicable that Mr. Glass would be able to afford to do things like this, especially since he experiences routine, high medical bills. Then it makes sense when you realize Mr. Glass is actually a criminal mastermind. The business is partially just a front, and partially just a hobby. He can be as picky as he likes with his "business", because it has nothing to do with why he has money.
Alternately, it seems insane because he is insane. One can hardly expect a supervillain to act rationally.
If you've been around serious artists, he attitude is not that out of normal. They tend to take themselves and their artwork very seriously.
And it's not as if he's lost anything permanently by turning down one sale. He's a good salesman and could surely find a buyer who will take the same piece of work seriously.
Although the villains are associated secondary colors, David himself wears a green poncho. This is probably because, even if he doesn't realize it, he's just a pawn in Elijah's mission.
There have been numerous accidents in history with only one survivor, this doesn't prove the survivor is a superhero, as the villain in the story would like to believe.
Mr. Glass knew this. Being completely uninjured by a train wreck, displaying mild psychic aptitude towards human iniquity and the ability to perform feats of superhuman physicality are probably what set him apart from the rest.
Paper cuttings in his office during the reveal show that he is aware of this. Such as the one referencing a lone survivor of a mud slide.
Elijah gave David a fairly thorough grilling about his Nigh-Invulnerability in their first meeting along with a healthy dose of skepticism at the start, which suggests that David may not be the first person approached and there is a long list of qualifications besides simply surviving a disaster. Elijah had the idea but it took the entire length of the movie for both to become convinced.
Elijah sets out two specific criteria he was looking for: A lone survivor, and miraculously unharmed.
David has the power to sense people's wrongdoings just by touching them. Perhaps this explains why he's drawn to his wife? He is shown being very intimate with her on several occasions (lifting her from the car wreck, carrying her to bed, nuzzling her after a "nightmare"). He never sees any misdeeds, so she must be ethically pure and she complements his own sense of justice.
Or that he loves her despite any misdeeds, as a deleted scene strongly hints that she either was having or was considering having an affair.
Assuming having an affair is the caliber of misdeed that rates a superhero's notice. Superheroes aren't generally known for championing cuckolds.
This would actually explain in part why David feels withdrawn from his family, he subconsciously senses any bad thing they do. He only actually touches Audrey twice in the present day, once briefly holding hands with her at the hospital and at the end carrying her to bed and hugging her. The first was hesitant and awkward, the last very affectionate. For him understanding that power lets him know why he feels that way around her and he finds a way to forgive her.