She has the right to change her mind.
And it’s not “too little, too late” - on the contrary, it’s just in time.
Again: Cornwell listened to reason. She realized she was wrong. She didn’t commit genocide, she averted it. And she deserves recognition for that.
First off, you don't get credit for changing your mind on committing genocide. That's what you're supposed to do. Also, it wasn't some stray thought that she immediately dismissed, they went through with planting the bombs.
Second, "averted" implies that she prevented someone else's attempt to commit genocide, not just that she paused her own. You're treating it like she heroically stopped an atrocity from happening, not that she was actively behind that attempted attrocity.
Third, Starfleet Command didn't totally change their mind, they just passed the buck to L'Rell, and let her hold the Klingon Empire hostage in order to undertake a coup and put herself in power. And if they had called her bluff, she had the option of still going through with it.
Fourth, Cornwell's right back with "the ends justify the means" bullshit in this episode, so clearly she didn't learn a damn thing from her aborted genocide or Michael's big speech about reaffirming Starfleet's values.
Fifth, go plant a nuke in Washington DC and then say "psych!" and see how well that works out for you with the FBI, with or without using the nuke to blackmail a favorable leader into total control of the government.