Let’s say Omni Man had a change of heart before killing the Guardians and decided to warn Cecil and the heroes of Earth that Viltrumites are coming. Would Nolan, Mark, Cecil, the Guardians and the rest of the heroes be able to stop the invasion? Assuming the Guardians are as competent as they are in the show and have prep time.
No, The Guardians weren’t strong and powerful to defeat a viltrumite like Nolan they would only stand a chance against baby viltrumites like Mark