Even in your image Zeref said END is the strongest demon he created, and he hoped that END would be resurrected by his conversation with Mard. No, valuing something made him weaker, which his Death Curse still affected something as powerful as Aldoron, at full power it would be a stomp, Ultear was merely a retcon then since she must have used hax, not natural durability, he can one shot someone like Zancrow anyway. That did not mean it wouldn't spread to the rest of Aldoron. Assuming Aldoron can. Zeref scale to Phoenix who can end the world.
Yes, END is the strongest demon Zeref created. That's not saying much, considering none of the etherious are anywhere close to Zeref's level. He never told Mard that he hoped END would be resurrected in their conversation. He never even tried to help Mard. At best he said that Mard tried.
There's no retcon. This has been the way Zeref's curse worked since the beginning, and still the way it works now. When Zeref feels that life is precious, his curse randomly kills people and weakens his magic. When Zeref forgets the value of human life, his curse no longer kills people and his magic gets stronger. The death curse doesn't get stronger when Zeref stops caring about life. It flat out stops happening. That's why everyone in the Alvarez Empire was safe from Zeref. He didn't care about them.
The curse only fired off against Aldoron because Aldoron was precious to the Wolfen god seed. This was even explained in the actual chapter.
The real Zeref couldn't care less about Aldoron, so the curse would never even trigger. Even if for some reason Zeref started caring about Aldoron, he
still wouldn't be able beat Aldoron because the curse will nerf his regular magic to the point where anyone that is stronger than Ultear could simply knock Zeref out. Aldoron just steps on him.