I have installed mod to show to-hit-chance but this looks beyond crazy.

Yes, due to how chance-to-hit is calculated. Roughly speaking, MRK determines the base accuracy without additional AP for aiming, while the bonus for additional aiming is determined by the weapon's aiming bonus (which is reduced if merc has low DEX). Every low MRK, high DEX merc (like MD and Livewire) will routinely headshot everything not behind cover with a modded M14 or Gewehr 98. Give them an AK-47 for that classic JA2 feeling :)