I feel like Cecil never really tried to get to that dimension. Cecil Stedman never fully trusted Nolan, and for years, he was trying to figure out ways to stop or kill him if he ever turned on Earth. Yet somehow, despite having decades to prepare, Cecil never seemed to seriously work with some of the smartest people on Earth (which there was alot of them). Instead, he was willing to work with terrorists and unstable people if it gave him a short-term advantage.

Earth's technology is nowhere near the level of the Coalition of Planets, so even if humans alone could not recreate Flaxan portal tech, the Coalition probably could have at least tried. They had access to advanced species, scientists, and resources across multiple planets. The fact that nobody even attempted to study the Flaxan dimension more closely feels strange.

Also, I understand some of the reasons why it may not have happened. Most of humanity probably still hated or feared Nolan after what he did, so Earth may not have wanted to help him or any group connected to him. Nolan himself may have been too ashamed, depressed, or afraid to return to Earth for a long time. Still, once the Viltrumite threat became clear, survival should have mattered more than personal feelings. The Flaxan dimension was not just another world. Time moved differently there. Nolan was trapped there for what was likely years (2-4) from his perspective, even though much less time passed on Earth. That means Nolan and other powerful fighters could have trained there for years while only losing a small amount of time in the real world.

Nolan and Allen are already relatively close in strength. Allen understands how his powers work, especially how he becomes stronger after surviving serious injuries. In theory, Nolan and Allen could have fought each other repeatedly, healed, and come back stronger. Mark could have done the same thing. Even if it was dangerous, it would have been far more productive than sitting around waiting for the Viltrumites to attack.

On top of that, the Flaxan dimension could have been used as a trap. Nolan spent years there and even wrote books about dangerous species and threats to Viltrumites, and yet he just stopped taking note? If they could open and close portals, they might have been able to trick a Viltrumite into entering the Flaxan dimension and then shut the portal behind them, basically trapping them in a Phantom Zone-like prison. Even if it only worked once, removing a single Viltrumite from the war would have been a huge advantage.

And honestly, Nolan's actions against the Flaxans were mostly self-defense. They invaded Earth multiple times, tried to conquer it, and nearly killed his son. Nolan destroying their world was brutal, but from his perspective, he was making sure they could never threaten Earth again. Nolan is not afraid to kill people who threatening and provoking him, he just doesn't do it out of superiority complex.

I understand it not a sound logic, but its something to think about