For the first one, you want to check
Code:
if(attacker)
return PLUGIN_CONTINUE
because the attacker will always be worldspawn if they fell (index 0)
Also, for the headshot thing, you can get the origins of both players and then subtract the attacker's origin from the victim's origin, multiply the vector by -1, add a few units to the z axis, then set the victim's velocity to that.
For the "make his head fall back" part, there's probably a sequence somewhere for that. You'll have to load up HL model viewer because I don't know any of the sequences off the top of my head. But to actually implement it,
Code:
entity_set_int(id,EV_INT_sequence,num)
__________________