When Red and the Tethered versions of the Wilson family invade the house, Red tells a story about a girl and a shadow. She talks about how, unlike her, the girl got warm food and presents on Christmas. We later learn that the Tethered were trapped underground by their link. So how did she know about the world above ground? It's because she's actually the real Adelaide, who as a child was forced underground by her Tethered counterpart.
Adelaide knowing about and explaining the Tethered's Kill and Replace plan to her family makes a surprising amount of sense, considering the Adelaide we've been following for the whole movie is a Tethered.
The poster depicts Red looking out from behind a mask of Adelaide's face, in reference to the Tethered's goal to Kill and Replace their counterparts. It's also foreshadowing the fact that Red was the original all along, which is further foreshadowed by her crying.
Another poster merely consists of scissors in someone's grip. The gloved one is covering the ungloved one, possibly foreshadowing the reveal that the Adelaide we've been following the entire time is the Tethered. Bonus points for the ungloved hand being the one to actually hold the scissors.
The Tethered's weapon being golden shearing scissors. In Greek myth, the Fates cut the thread of a person's life - what tethers them to this world - with scissors. The Tethered want to kill their originals to destroy the Psychic Link, hence cutting the thread, making this especially meaningful.
Also the scissors are symmetrical, with the handles resembling two heads back to back made of two identical pieces bound together to move in tandem. A very apt icon of the Tethered and their nature.
Jason's Ambiguous Disorder behavior is because he's half tethered. If Red's story about her children being born odd is true, then it implies that those born with Tethered genetics are born with high chance of behavioral issues or disorders.
It makes sense why the Tethered were considered a failure if Red's theory on them being made by the government is true. They're too dependent on others, judging from the influence they receive from their counterparts and electing Red, the original, at an implied young age to be their leader. It's made clear from the first Tethered appearance at the end that he's been standing in place for an entire day without any food or water. Barring the fact they seem to have a natural desire to Kill and Replace their real selves, it would be especially difficult for the government to pull a Kill and Replace with a clone who needs 24/7 supervision especially one that can't speak or talk like a normal human being who would needed to have to be taught.
The man was standing in place because he was the Jeremiah 11:11 Tethered. It looks as though Red commanded each Tethered to take their place in the Hands Across America recreation once they killed their counterpart.
Notice how Jason's Tethered counterpart is named Pluto. Like his sister Umbrae (Tethered Zora), Pluto's name sticks to the theme of shadows and darkness (Umbrae means shadow) as the dwarf planet Pluto is the furthest from the sun and gets its name from the Roman god of the underworld. Pluto is also the name of a well known Disney character. Pluto the dog walks on all fours in contrast to the other animal characters in the Micky Mouse cartoons. Similarly, Pluto is the most animalistic of the Tethered, shown several times walking on all fours himself.
The names of Red gave her family also make some sense when you realise that she's the real Adelaide. Abraham would be a reference to Abraham Lincoln, one of the most iconic US presidents and no doubt someone Adelaide would have learned about before being replaced. Pluto, as mentioned above, is the name of a Disney character as Adelaide could have been a fan of Disney cartoons. Umbrae's name however is the odd one out as Adelaide wouldn't have knowledge in Latin unless by some strange miracle she is naming her after the home decor company Umbra from Toronto, as the company opened in 1979 and she was replaced in 1986, giving her enough years to remember the name of said company.
In the climax, Adelaide enters the Hall of Mirrors and immediately heads toward a hidden door in the wall that hasn't been seen before. Of course she knows about it, she's actually the original Tether and came out that door when she swapped places with the real Adelaide.
It should also be noted that although we see a bunch of doors and other passages in the underground, Adelaide doesn't even check any of them, but heads straight to the Tethered base.
The Tyler's Tethered don't immediately attack the Wilsons or tries to murder them. This is because they must be friends with their version of the Wilsons thanks to the Tethered's Psychic Link and with Red, being the leader of Tethered society. They would know how important killing their counterparts would be to their friends.
When Adelaide kills Red, she turns downright feral and even after impaling her and then going the extra mile by wrapping her handcuffs around her neck till it snapped. It also the last sign outright telling the audience that Adelaide was the Tethered the whole time. So far with all of the other tethered whose not the Wilsons are shown to waste no time in killing their counterparts as shown on the news report and with the Tylers however Red wants her counterpart to suffer before killing her as shown in their final fight where she obviously could have finish Adelaide at anytime unlike Adelaide who waste no opportunities in trying kill her. A clone created to Kill and Replace a person would have a natural instinct to quickly kill their target so they can take control of their life.
Related to it one might wonder as to why Red was toying so much with Adelaide, considering that the entire plot was kickstarted by her wanting revenge on her clone, but when you think about it, this may very well be part of the revenge, Red wants Adelaide to become feral, in a twisted way to show both of them who is the real one, aka. The one who doesn't get an unnatural urge to kill the other, in short, Red wants to unmask Adelaide as the true monster all along
When the family is eating together Adelaide is eating strawberries while the rest of the family are eating fast food, demonstrating how she is not like the rest of her family
After having been forced to eat nothing but raw rabbit meat for the first ten years of her life she's probably a vegetarian
The Itsy Bitsy Spider is relevant to the plot when you think about events from Reds perspective. She gets out of the underworld (climbs up the water spout). Then, shes lured back down there by Adelaide and nearly beaten (down came the rain and washed the spider out) but then Red kills Adelaide (Out came the sun and dried up all the rain) and then Red returns to the surface (The itsy spider went up the spout again).
In the last fight Red gracefully dodges Adelaide's attacks and slices at her, a marked contrast to the other Tethered's fighting style, running at their victim and stabbing them in the head or neck. This is because Red is a normal human, she can't take the damage a Tethered can. She also knows she can't take Adelaide out in a single stab, so she weakens her before attempting to land the killing blow.
The Tethered specifically using rabbits as their sole source of food makes some sense. Rabbits are infamous for being notoriously fast breeders, meaning that a large rabbit population can provide a theoretically endless supply of food.
Red's line of "We're Americans". makes perfect sense. They were created by a United State (of America) cloning project that uses American citizens to be used, ostensibly, by the United States for control. They were literally created by Americans for Americans.
The reason the real Adelaide is behaving like a Tethered throughout the movie is because she was trapped in a hellish world for over 30 years, it drove her completely mad. Being cooped up underground with no real human contact made her animalistic and psychotic. She was so insane that she convinced herself that she is a Tethered.
Josh Tyler's Tethered counterpart Tex, while still terrifying as the rest of the Tethered, also comes off as sorta comical and even a bit cartoony. He shares a name with well-known animator Tex Avery, and Tex Watson - the right-hand man of the Manson Family.
The fact that the kids are technically half-Tethered due to being born of Adelaide. Whether that will cause any issues down the line in their life has yet to be determined.
Why is Reds voice so hoarse? Because when the Tethered!Adelaide choked her Untethered counterpart, she likely crushed her throat and permanently damaged it.
Alternatively, her voice is just hoarse from lack of use after so long among the non-vocal Tethered.
Remember when Red talked about that creepy fairy tale? How the princess met a prince and had kids? And her shadow met a brute and had monsters? Considering their Psychic Link, how do you think they came together and had kids?
Take this another step further what does the very existence of the kids imply? if we take Red's story at face value then the kids aren't clones, but rather the "mirroring" shown in the Tethered goes all the way to reproduction, meaning that when the two Tethered come together in the same way as their "original" counterparts then they make the exact same kids, with the exact same link. If that's the case how many generations of Tethered have there been since what ever government facility abandoned them? Has the government even been monitoring them, or did they assume they couldn't reproduce and would die in a single generation?
When the Wilsons drive up to their burning car, they don't notice Red sneaking up behind them. Was she simply planning to to grab and subdue the closest one, or did she know that Jason had figured out their mirroring and let her son die so she could capture Adelaide's?
All of the rabbits will most likely die from starvation in the underground because all the Tethered moved above and there is no one left to take care of them. The Tethered have no reason to go back below for their original food source, so unless the rabbits have access to food and water, they will most likely all die.
It seems like the Tendered can easily track down their counterparts due to their psychic link. If that's true, then there is no place on earth to hide from those monsters. You can have plastic surgery and change your name, your clone will still be able to find you.
After meeting up with Red in the classroom Red spends several minutes explaining the origins of the Tethered Why does she do this? Both Red and Adelaide know all of this given the fact that Adelaide was the Tethered one all along. Nothing Red says should be much of a revelation to her.
It seems possible that Adelaide may have repressed bits of her past.
Adelaide was only a child when she switched places with Red. While she may have understood how horrible the underground was she may not of understood why she existed. Red could also be using this as a "remember who you really are" Breaking Speech.
The Tethered eat nothing but rabbits, but what do the rabbits eat? For that matter, if The Tethered truly are an abandoned Government project and thus no longer receiving funding or oversight, who or what is supplying them with exact or close-enough replicas of their human counterparts' clothing? Where did they get that many matching red jumpsuits, fingerless gloves, and fancy pairs of scissors?
Not sure about the rabbits, but it's been suggested that the scenes where the Tethered are wearing the same clothes as their regular counterparts are just how Red pictures it and not what it actually looked like.
The Tethered badly break Gabe's leg upon breaking into the house. He's shown limping several times, but other than that, it never affects - or it seems, even clouds - his ability to perform extremely brutal stunts such as killing Abraham (especially on the water!) or Tex. Sure, it's a shout out to Funny Games, but just seems to create unnecessary plot holes.