[@BloodyRed]

Red can still throw his punch but note that the *KRIK* in my description, or the sound effect of Red's thumb being broken, occured after the half-step.

Studying your post, I see that Red takes the half-step and then slides parallel with Kull, using his momentum to pull Kull into the punch for added power after locking out his arm. Thus, taking that order into account, Kull's arm would ultimately not be locked out if he broke the thumb, and he'd not take the punch at full power if Red carried through anyways.
I'm not saying Kull won't get hit, but his move kinda interrupts Red's.

As for the heart stopper, here's a quote from the Wikia on that.

"...Impact occurring within a specific 10– to 30-millisecond portion of the cardiac cycle. This period occurs in the ascending phase of the T wave, when the ventricular myocardium is repolarizing, moving from systole to diastole (relaxation).
The small window of vulnerability explains why it is a rare event. Considering that the total cardiac cycle has a duration of 1 second (for a base cardiac frequency of 60 beats per minute), the probability of a mechanical trauma within the window of vulnerability is 1 to 3% only. That also explains why the heart becomes more vulnerable when it is physically strained by sports activities..."

Now it IS technically possible, but Red would have to be flat out superhuman to perform it at will, as he'd need the senses and utterly flawless timing to detect virtually everything about the state of his opponent's heart and attack at rhe right fraction of a millisecond. It's not really something any human in history has ever been able to do outside of a rare freak accident.