Writing doesn't work like that
There seems to be some confusion about the definition of foreshadowing. Foreshadowing is a subtle hint at future plot direction. When Ace makes first mention of
Blackbeard, that is foreshadowing. The audience was only informed that Blackbeard was a criminal Ace was tracking. His identity and role in the greater narrative of the story was not explained, leaving the audience free to speculate.
Luffy declaring war on Big Mom was a matter of telegraphing, not foreshadowing. Telegraphing is the direct, full disclosure of a future plot development. There are no remaining mysteries about Big Mom's role in One Piece. We know she poses an imminent threat to Fishman Island, that Luffy has declared war on her, that a bomb was inadvertently sent to her, and that Jinbe can not join the Strawhats until she is defeated. The only question is when the Strawhats confront her crew.
A red herring is the opposite of foreshadowing, a subtle hint meant to mislead the audience, obscuring the direction the plot is headed. A red herring is meant to manipulate expectations, increasing the impact of surprise revelations. The statements Law made about himself before the timeskip are a good example of a red herring. Part of the audience was manipulated into believing he would either be a villain or callous, uncaring character. His true role in the manga was established when he saved Luffy and Jinbe's life after Marineford. A red herring must be subtle, lying to the audience renders it ineffective. The disconnect between Law's words and his actions (a literary device called
irony) was enough to manipulate expectations without lying to the audience.