I want make my monsters be more accurate in choosing the line of fire so they would not fire if the way to target is blocked by their allies. What are functions that check where creature aim and if there is a way to check who is standing first in front of shooter?

I don't know if this might help you, but you can see how this works in the Strife progs more clearly.

You can use aiming functions and see at which actor it did aim at. Strife doesn't have this, it only prevents fighting back at alies when damaged.