What exactly makes the duo so powerful? Their strengths complement each other very well: Shiro, a reincarnation of a literal Robot Girl, can perfectly play by the rules no matter how complicated they are, while Sora, representing intuition and ingenuity, is able to adjust to changing circumstances and improvise a plan on the spot. This is very similar to how computers and humans, in tandem, can achieve better results than either of them alone, whether in board games or other more pragmatic pursuits. The duo's aspiration to become a literal god is nothing but The Singularity in human form.
Shiro and Sora generally suffer nervous breakdowns when apart from each other, so why did Shiro not suffer it when Sora was not present during Episode 8's and 9's Retgone situation? Because he was still there in a way except immaterial - she hears his voice and constantly feels that there is something else which was in fact Sora who followed her around while trying to get her to realize what really happened.
Furthermore, how they were able to be separated while playing the game in episode 10 and 11. They were holding hands in the real world; they don't care about the setting of the game world once they learn it's just a simulation.
Jibril's devotion towards Sora (and Shiro by extension) is not only because she bet herself and lost, but also because they're challenging Tet, an Old Deus. Challenging someone to a game is practically a war declaration in Disboard, so 『 』's intention of defeating Tet is the closest equivalent to killing an Old Deus you can get now that bloodshed is forbidden. Jibril has finally found a master who would allow her to be the god-killing Living Weapon she was meant to be.
It seems weird that Imanity's race piece is the white king; but if you think further, it actually makes sense. The king piece is the weakest piece as it can only move one square per turn, but it's also the most important and the whole point in Chess is taking over the opponent's king. Imanity is the weakest Exceed but thanks to Sora's and Shiro's arrival, Imanity is slowly becoming the axis of the whole chessboard that is Disboard.
It can also be Foreshadowing: if Imanity is the weak but indispensable king, it means that all the others 15 pieces are at it's disposition to serve and protect; Imanity will indeed take over the world and the other 15 Exceeds will be supporting them against Tet.
Tet possesing the black king piece actually says more than just being the One True God, opposing Sora and Shiro. Tet was the weakest of the Old Dei, but his survival earned him godhood.
The colours themselves are symbolic: the white player goes first (but supposedly black wins). So it makes sense that the 16 Exceeds are the white side and go for the attack, while Tet with his black pieces will defend but, presumably, win (he's the God of Games, after all, so there's no way he'd want to lose the ultimate game of chess).
Sora's coronation speech is right on the money. As the series progress, we see that although the other races now also rely on planning and knowledge, the backbone of their way of doing things, as well their trump cards, are still their physical and magical abilities. Whether it's magic cheating, or extreme physical endurance and knowledge gained through long life, or physics-defying martial abilities, the other races use their brains to supplement their brawns. Now they are being defeated by a pair of no-brawn siblings that rely completely on their brains.
A curious thing, but Steph winds up falling to the same trick twice, that being Exact Words. During her first game with Sora, she planned her strategy under the belief that Sora was trying to use the game to get a place to live. But he never said that, all he did was complain about money but he never said what he'd be playing for, something Steph herself realizes after she's already lost. Later when meeting the Warbeasts, Ino asks Sora if he's there to visit the embassy. Note what Sora says next. "You're pretty sharp." A statement that is not a direct confirmation or denial, just a neutral observation. And both Ino and Steph fall for it, with Steph thinking to herself that the Warbeasts really are psychic. That's why she's caught off-guard and confused when Sora reveals his real reason for being there, to challenge Elkia to a game. Ino, of course, falls to confirmation bias, since most political leaders would be going to a foreign embassy just for a visit and Sora's neutral statement didn't give away any indication of lying.
Shiro's distant personality is easy to explain as a child who can't fit in and is bullied for her genius. But what about Sora? How can someone who is a NEET and Hikikomori have so much charisma? It's the same reason Sora often acts like a Sociopathic Hero. When he's displaying his charisma, he's lying through his teeth and treating the entire thing as a game. All he's doing is answering what the NPCs want to hear so he can progress in the game. He is offering nothing, factual or emotional, on the line. He has so much confidence when he talks because he is risking nothing while getting ahead. This also played a role in why he became a Hikikomori.
This facet of him is exactly what made him open up to Shiro in the first place, too! Shiro was the first one to see past Sora's surface level charisma. The first one who could tell that even with a smile plastered onto his face, he wasn't actually happy with the people around him.
Sora and Shiro's reactions to being separated from each other mirror Riku and Shuvi's thoughts before their deaths.
Shiro locks up and goes catatonic. During Shuvi's ultimately futile battle against Jibril, she laments that it happened because she strayed from Riku's side.
Sora's cocky attitude breaks down and he starts apologizing profusely. In Riku's last moments, he lamented that without Shuvi acting as his Morality Pet, he sent the entire Ex-Machina race to their deaths and thus believed himself unworthy of becoming the One True God of Disboard.
Sora and Shiro's apperances. They're practically Riku and Shuvi but with their hair and eye colors switched. Sora and Shiro are how Riku and Shuvi imagined their hypothetical children would look like.
The consequences of Imanity losing their Race Piece and no longer being protected by the Ten Pledges are bad enough, but imagine what would happen to the human slaves of Elven Gard if their masters no longer have to obey the prohibition against violence. With this in mind, it's no surprise that Kurami and Feel felt the need to take action.
The series runs on the prospect that Shiro and Sora will unite the 16 Exceeds, collect all chess pieces and then challenge Tet to the ultimate game. But what happens if, after all the trouble they went through, they'd actually lose? In the case of the game, nothing less than risking those same race pieces would be appropriate when challenging God himself. A subsequent loss could easily mean the same as when Imanity risked its Race Piece, except this time all Exceeds would fall out of the protection clauses. A repeat of the bloody war from times long past would loom again on the horizon once certain forces realize they no longer have to behave. Tet would have unwittingly doomed his whole world and he couldn't go back on it if those were the conditions, either.
Sure he could; The Suniaster makes him OMNIPOTENT (at least within the sphere of influence of Disboard's Reality. He could technically break ANY of the Pledges, since they only hold any power as he's passively "enforcing" them via the Suniaster, which is infinite power, basically.
Plus the Race Pieces (as Sora and Shiro note) are something that shouldn't be taken away by nature. The way I see it, it's not that you have to bet them, rather you just need to have them to challenge Tet.
While the thought is terrifying to be certain, it's one very unlikely to happen. Tet's whole view on the world is that those who use inherent power to harm others for gain are thoughtlessly cruel, and no better than beasts. If anything, Tet might hope that a loss would inspire the world to try harder to defeat him. It would go against his own views were he to remove their race pieces just for beating them. The fun of games is that one can play them over and over again with each opponent getting stronger every time through loss and experience. Tet doesn't want one person to hold all the races on a noose and control them with violence. He wants an opponent who has proven themself through victory and cooperation to challenge him to the best game ever. And, let us not forget the Tenth Commandment. "Let's all have fun and play together!" The more people playing the game the merrier, his goal isn't to force a be all end all fight for the world. He wants people to have fun with each other!
