Yes, by the time of that cannon/gunpoint scene they already knew Eren was a Titan shifter. Earlier in the Battle of Trost they (along with Jean and a few others) personally watched him emerge from the nape of the mysterious Titan that had been fighting the Pure Titans. They were even put under orders not to talk about it. When the Garrison under Captain Woermann later pointed cannons at Eren, Armin, and Mikasa and fired, Reiner, Bertholdt, and Annie were already aware of what he was; the partial transformation they saw when they rushed over the wall was confirmation of something they’d already witnessed, not a first reveal. They weren’t the only ones who rushed either—Jean went with them—so it reads more like nearby cadets reacting to cannon fire and Titan steam inside the walls than secret prior knowledge from before the battle. Their real interest as Warriors kicked in once they realized another intelligent shifter existed (potentially the Attack Titan or a clue to the Founding), which is why they later discuss using or protecting him while he plugs the hole. Before his first transformation that day, though, they had no idea. The reply claiming they didn’t know yet is simply off on the timeline.

Eren didn’t need full control of the Founding Titan or to physically go back in time. When Zeke (with royal blood) pulled them into Paths to view Grisha’s memories, the Attack Titan’s unique ability to send and receive memories across past and future inheritors let Eren “livestream” his perspective and words straight to Grisha in that moment. Paths ignore normal time, so while Eren and Zeke were watching the cave scene, Grisha was simultaneously receiving those future memories of Eren urging him forward. That’s why Grisha could hear Eren’s voice and see the visions of Faye, the Restorationists, and the “scenery” of the Rumbling even though he was hesitating. Eren was essentially injecting the exact motivation Grisha needed at the critical second, creating a closed loop where the future Eren ensured the past events that made him possible in the first place. He already knew this would happen after kissing Historia’s hand years earlier.

Rod Reiss refused to consume Eren and reclaim the Founding Titan himself primarily out of fear and self-preservation. As a member of the royal bloodline, inheriting the Founding Titan would have subjected him to the First King Karl Fritz’s Vow Renouncing War, which forcibly overrides the will of any royal holder and locks them into a pacifist ideology (as it did with his brother Uri and previous Reiss family members). On top of that, he would have been bound by the Curse of Ymir, limiting him to only 13 years of life after inheritance. Rod was a coward who preferred to stay human, avoid the vow’s control, and manipulate events from the shadows rather than become a short-lived puppet of the First King’s will. Historia was specifically required because only someone with royal blood can fully access and command the Founding Titan’s true powers—Eren, despite holding it, could not properly use them on his own due to lacking that bloodline. Rod’s plan was for Historia to inherit the Titan (by consuming Eren) so its full potential could be unlocked under his guidance, allowing the family to reclaim control while he remained free of the vow and the curse. When she rejected this, he tried forcing the issue by turning her into a Titan, ultimately leading to his own desperate and failed transformation.

He didn’t actually know—there was no prior example or rulebook for Titan inheritance because Ymir was the first. After she died taking a spear meant for him, Fritz was just a desperate, power-obsessed king terrified of losing the one thing that had built his empire. In a world that already had ideas about consuming someone to take their strength, ordering his own daughters to eat their mother’s body was a brutal, last-ditch gamble rather than informed strategy. It only “worked” because Ymir’s slave mentality still bound her even after death; in the Paths she continued to obey him, so the power transferred when they consumed her. That same desperate order is also why the powers later split and why pure Titans instinctively try to eat people—they’re chasing a transfer method that only exists because Fritz forced the issue in the first place. The manga never gives him some secret ancient knowledge; it just shows a cruel man throwing the most extreme option at the wall so the power wouldn’t die with her, and the Coordinate (and Ymir’s lingering obedience) making it stick. Classic Fritz.

When Reiner and Bertholdt revealed themselves on the Wall, Bertholdt’s Colossal Titan didn’t produce a full-scale explosion because Colossal shifters can deliberately control the intensity of their transformation’s heat, steam, and kinetic blast. In that scene he only did a partial transformation (essentially forming from the waist up while remaining attached to the Wall), which generated a strong gust of wind and continuous scalding steam to keep the Scouts at a distance, but not the massive detonative release he later used at Shiganshina. This control trades off raw explosive power for better muscle density and precision, so the transformation stays destructive enough to disrupt the Survey Corps without vaporizing everything nearby. That restraint was also practical. A full nuke-style blast right there would have risked killing Reiner (who was already fighting Eren), Ymir, and any Titans Annie had summoned, while also potentially destroying Eren—the very person they needed to capture alive. By keeping the output lower and then later dropping and evaporating his Titan form in a controlled steam burst once Reiner signaled, Bertholdt created the opening for them to grab Eren and Ymir and escape without turning the entire area into a crater. It’s one of the clearer demonstrations that the Colossal’s “explosion” is a tunable ability rather than an automatic side-effect of transforming.

Ymir didn’t fight the Marleyans or use the Jaw Titan to save herself because she chose not to. After spending roughly 60 years as a mindless Pure Titan, she only regained her humanity by eating Marcel Galliard during the Warriors’ mission to Paradis. That act left her feeling deeply indebted to Reiner and Bertolt—the very people whose arrival gave her a second chance at life. When the time came, she willingly went back with them and let Porco Galliard inherit the Jaw Titan, both as a form of repayment and to protect Historia from further pursuit. Fighting would also have been pointless. She was already in their custody, outnumbered by trained Warriors with multiple Titans, and had no realistic way to escape Marley alone. More importantly, her core motivation was never pure self-preservation; she had resolved to live on her own terms, and in the end that meant sacrificing herself so Historia could keep living freely. Her earlier ferocity as the Jaw Titan was always tied to protecting the people she cared about, not clinging to her own survival at any cost.

Spinal fluid doesn’t automatically hand out unique Titan powers to everyone who drinks it—most of the time it just turns a Subject of Ymir into a mindless Pure Titan (especially with Zeke’s fluid + his scream). Eren got hardening from a specialized “Armor” serum (basically concentrated spinal fluid from an Armored/War Hammer-type source) while he was already a shifter; it added a trait rather than giving him a whole new Nine Titans power. Falco’s wings are a one-off hybrid situation: he ingested Zeke’s Beast Titan fluid, became a Pure Titan when Zeke screamed, then ate Porco and inherited the Jaw Titan. The leftover Beast influence (plus memories of a past flying Beast Titan) twisted his Jaw form into that avian version. Full powers only transfer when a Pure Titan consumes a shifter’s spine, and there are only nine of those—so random people chugging fluid just end up as ordinary Titans or get minor trait bleed, not their own power.

Eren cut his hand before meeting Armin and Mikasa in Marley as a calculated precaution. By that point he was fully committed to the Rumbling and knew the conversation would turn hostile—he was about to deliberately wound them emotionally by calling Mikasa a slave and rejecting their bond. A fresh injury meant he could transform instantly if either of them tried to restrain or attack him, turning the table into a quiet threat rather than a reunion. It doesn’t contradict his broader goal of protecting them. Eren’s version of protection had already become extreme: he wanted to push Armin and Mikasa far enough away that they could oppose him without hesitation, survive the conflict, and ultimately be remembered as the ones who stopped him. The cut hand was just the physical expression of that same cold resolve—he was ready to defend the plan even against the people he still cared about.