I have become really curious as to why Oda-sensei has decided to leave the “heart-stabbing” of Monet in such an ambiguous note. Firstly, Caesar’s stab was quite shallow and only managed to penetrate a few centimeters in the ‘block of flesh’. Secondly, there is more to consider as to whether what Caesar stabbed really was Monet’s “heart”. The fact that the “heart” was contained inside a ‘block of flesh’ and that the stab was shallow suggest that Oda may have left Monet’s “death” on an ambiguous note on purpose because he may have intended to cover her later in the story, be it through the cover page story or the general story of One Piece.
Now there is no question that what Caesar stabbed was that ‘block of flesh’ and that blood came out indicating injury to it, but as can obviously be seen, Caesar is in a weakened state and the position he stabbed the ‘block of flesh’ in is such a weird one, only a small fraction of his remaining strength would have come through in the stab.
Furthermore, the panel below the one showing Caesar stabbing the ‘block of flesh’ shows Monet with a huge square hole in her chest, which indicates that not ONLY was the heart taken out but the surrounding flesh as well. The scene can be interpreted as the shallow stab by Caesar only managing to penetrate the flesh surrounding Monet’s heart and not go deep enough to actually hit her heart. And as we all know, flesh does bleed.
And as we can see from this panel, the ‘block of flesh’ does not seem to be releasing blood anymore and from the lack of blood coating the weapon used by Caesar shown, the stab is implied not to be very deep:
The resulting force from Caesar forcing his hand down onto the ‘block of flesh’ likely caused an impact on the heart of Monet’s which resulted in knocking her out and Monet’s weakened state could have attributed to her coughing up blood. Law was knocked out (and coughed up blood) when Caesar applied force to his heart when he was “caught” so Monet getting knocked unconscious because of the force applied as opposed to her heart being penetrated doesn’t seem that unlikely.
The ambiguous nature of the scene where Caesar stabs the ‘block of flesh’ containing Monet’s heart can include the interpretation which I have just laid out. Concluding that Monet’s death is a certainty at this point in the manga seems a bit hasty and impulsive to me. At the very least there is a good possibility that she could have be alive.