To be a bit more serious in my response. I pretty much covered why I do not think her death is guaranteed. I am not even considering Oda's philosophy. I pointed out why the sequence seems good as a cover up panel by panel. That's not even taking into account that even without her heart she was able to have the properties of logia users. But some of these reasons have been discarded as "Oda is not perfect".
If you want to analyze the panel further, first we have Clown face almost about to pass out, saying he cannot move, in a last desperate effort to get his way, he stabs the heart. You see his hand shaking, almost as if he could not hold the stake much longer. But to then say, and off panel he lifted his hand again and stabbed the floor thrusting into the ground before he could pass out in peace seem overly complicated. I am saying this as someone that proposed the same explanation a year or so back. It is not that it is impossible. It is just seems oddly specific for an off panel thought.
It is possible, but the same way it is possible that he only stabbed once, and as he attempted to dig deeper into the heart, the stake slipped off from the jelly like substance the cube is made off. And because he was trying to dig deeper when it slips, It gives it enough momentum to get stuck on the floor.
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@SeaOfHope:
Not to mention being trapped in a collapsing and exploding laboratory during that time frame with a killer gas fossilizing everything to oblivion.
The same fossilizing gas that Kinemon got freed from by falling over.