How often is kinda like it depends. But I mean, it can happen, my hands aren't always perfectly dry, I might even forget to dry them properly. I try to be careful now, but it can happen.
So, like 10-20% of the case is not under, I can't push it entirely back, but I could push it back some more (and even then, drops of water don't always fall perfectly vertically, so movement of hand can throw them under the desk anyway).
No cloth mousepad, I don't even use a mousepad.
I am not using any top fans, no, only front and back (and bottom if we include PSU fan). Thought the vent on top could also help with airflow though, not sure.