Flashlight and power validation

"screaming and acting sick as plague but not getting sick"

This IS server validation.

You see them get sick on the client side, by way of them coughing and spluttering, then the server validates it, and they're not sick.

I can't say if it works for the other things or not, would need to test it or be given the scenario details, but what is typically described with the Plague's power is an example of how server validation works. First you get the indication that it's happened, then almost immediately it's apparent that it hasn't happened.

Now I think of it however I expect the Doctor's power is simply latency. I've often hit people with it, they've screamed, and THEN still vaulted, but only when it's within about 1 second of screaming. But afterwards, their madness level has still increased, so they did get hit, they just got hit later on their end than on mine.

If they start their vaulting animation (even when you can't actually see it starting) and they scream, they usually will ignore it and vault anyways.