...
1) If you look at a certain timestamp, e.g. the 60 second mark as in your example, the y-value of the curves will tell you the probability the M4C will have killed the Panther (i.e. out of all shots fired 6 penetrating hits have been scored) within 60 s or less. In this case, the AP Sherman at 40 m (blue line) will have a kill probability of a bit over 50%, while the HVAP Sherman (red line) will have killed the Panther in over 70% of all cases. Hence, the HVAP Sherman is more likely to knock out its target within 60 s.
...
Thanks for the math. Somehow it makes very clear why the 76mm overall isn't worth it. If you look at it, you'll see that you need 51 seconds for a 50% chance at 40 meters to kill a Panther with the shell that is meant to kill tanks and does next to no damage vs infantry. Meanwhile Panther needs 20 seconds to 100% destroy a 76mm Sherman at 50 meters. Even factoring in population values doesn't change the outcome (you get 1,5 76mm Shermans for 1 Panther). It just closes the gap a little bit, but it is still far apart.
All this AT medium tanks are just only made for fighting PZIV and don't scale at all into lategame for fighting Panthers. This way you'll always take the Jackson instead which can utilize its range advantage to win vs a Panther. As long as AT medium tanks like 76mm (and E8) suck at killing infantry there will be no reason for their existence. This is true especially outside 1vs1. 76mm/E8 should be straight upgrades of M4A3 with somehow equal AI power but some additional advantages like better penetration, health, accuracy, armor or RoF. If you want an example: It works out well for T34/85 beeing a straight upgrade of T34/76.