The Panther has 10 more range, it can easily fight an IS-2 until that one gets vet 2. At which point it becomes a roughly even fight slightly in favour of the IS-2 with a TTK of 53s vs 58s, though this is disregarding accuracy, which is going to be in favour of the Panther.
Tank classification has no universal parameters and completely depended on the owning army's own parameters. Its RL classification serves no point in these discussions. It was reclassified a medium within a year of service anyway.
I am not going to argue about the 10m range advantage at vet0, but in a duel the IS-2 should win by a decent margin. Especially when the range advantage is gone with veterancy.
I am also sure your numbers are wrong. Even with the classic TTK and neglecting accuracy I get 73 secs for Panther to kill the IS2 and 43 secs for IS2 shooting the Panther. At vet3, Panther needs 55 secs to kill the IS2 and 33 the other way around.
The IS2 has better rate of fire than the Pershing, especially at vet, needs a shot more and has way better armor. The downside in a direct duel is having 3-6% less pen chance vs the Panther.
While I think the overall accuracy will slightly favor the Panther, I assume the real advantage will be about 10% (maybe 15%) more hits than the other way around due to Panthers high scatter.
But even if you count all "scatter shots" as misses, which is the best possible calculation for the Panther, the IS2 still has a (very slight) edge in the TTK times.
This is a myth. The Axis do have a few outliners (Tiger II, Jagdtiger and Elefant), but otherwise the Allies are just as stocked when it comes to medium to heavy armor values. The only difference is stock versus doctrinal availability, but that is why the Allies get 60 range high pen TDs while the Axis do not. See below.
I'd like to point out though that Allies face Axis high armor absolutely regularly and have to plan for that every single game, while as Axis you only have to fear high armor units on a regular basis when playing vs Brits. Everything else is doctrinal and often limited to one unit. This context matters a lot.