I think your problem lies in needing a little bit of background/setting to anchor these bits of narrative in the storytime reality. Another problem is that those two paragraphs aboaut the golden key cover the same territory too much. Look at this rearrangement below. Does it flow any better?

“The key to my success lies within,” Jack Steadman always said when questioned on his powers of persuasion. He used to tap his chest and wink.
[some sort of setting or situation segue here]
“Use a knife, Bill, find the key. It's yours.” He had said, tapping his chest. Then he died.
As usual I was unable to resist any demand Jack made.
I guess I was as surprised as any when I cut into his heart muscle and came across the golden key embedded within the wall of his left ventricle.

“The key to my success lies within,” Jack Steadman always said when questioned on his powers of persuasion. He used to tap his chest and wink in my direction.
After slipping in the kitchen and cracking his skull, he had looked up at me with glazed eyes.
“Use a knife, Bill, find the key. It's yours,” he had slurred. Then he had died.
As usual, I was unable to resist any demand Jack made. Within five minutes of his death, I was covered in his blood. I was as surprised as any when I cut into his heart muscle and came across the golden key embedded within the wall of his left ventricle.
I admit it, it must have looked suspicious when Mrs. Warburton walked in--she screamed blue murder.

I think the problem with the first 13 is that you are trying to fit too much into 13 lines, which shouldn't be a problem in the actual story. I'd like to see an actual scene between the two characters before the death of one of them, and I think we need a better cause of death than "He slipped." Jack's power makes him cocky. Maybe it also makes him careless.

quote:

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In fact, humans are so hard to kill that you are going to have your work cut out for you to find a believable way for Jack to know that he is dying, be able to talk, and not be in a hospital.

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Degenerative disease, perhaps? Many examples in history of 'famous last words' do show some awareness on the part of the person dying that death is near, even if not the dramatic five-seconds-later scenario.

Or, a preluding scene (or a flashback) where Jack alludes to the key being so literally in his heart might work. Another alternative still might be that the MC inherits a letter directly as a circumstance of Jack's death, which tells him where the key is.

Another thought that struck me, though I might be wrong on this - gold is a soft metal, so depending on the realism and purpose of that key, it may be ineffective as an actual key(?).