Event-specific hit validation
I'm convinced that the general hit validation system that according to you has been implemented into the game months ago does not actually function. Hit validation has one job and one job only: overrule a client hit if it is deemed erroneous on the server. The fact that overruled hits practically never happen even if the killer player has a ridiculously high ping and as a result lands outlandishly far and late hits from the survivors' perspectives is evidence enough that it does not work, or might as well not. I have experienced and seen tons of game scenarios where survivors still get hit by all the impossible laghits they've always done, and I have also tested this in custom matches by manually inducing latency. The effect of latency is the exact same it has always been, with regards to hits: the more latency there is in the connection(s), the more ridiculous the hits for survivors. Even when I got as high as 300-400ms as killer, I never experienced an overruled hit.
With the hit validation specific to pallet stuns that is currently being tested on the PTB on the other hand, I was able create instances of overruled hits consistently.
So given that you seem to have managed to make stun-specific hit validation work, but cannot seem to make general hit validation work, my suggestion is for you to try and at least implement more event-specific validation. Dead Hard can be validated for much in the same way that stuns are: if the server receives client A's Dead Hard trigger prior to client B's hit event, in the according time window of Dead Hard's invincibility duration, that would be an erroneous hit. Hits on pallet and window vaults should also be able to be validated in a similar way: How far away from a window or how far into a pallet vault does a survivor have to be in order for it to be impossible for the killer to hit them anymore? Take this distance, and then take the time required to traverse that distance, given the respective vault actions. Let's call that time T. If the server receives client A's vault action at time X and client B's hit event at time X + more than T, the hit is erroneous.