Which proc's its effect first? Is the copilot crit nullified or can I turn one of the forced "normal" hits from the HLC to a crit after the shots are resolved.

You roll with an HLC then you immediately turn any natural crits to standard hits before applying any dice modifiers from either you or your opponent.

After this your opponent and then you may apply any normal modifiers such as Merc Co-pilot or Etahn, calculation or any re-rolls which may result in crits as normal.

Thanks!

Just remember the proper sequence for these steps in the combat phase:

Attacker Rolls Attack Dice (HLC affect to crits triggers here)
Defender Modifies Attack Dice
Attacker Modifies Attack Dice (Merc CoPilot would activate here, along with other things such as those mentioned above)

Defender Rolls Defense Dice
Attacker Modifies Defense Dice

Defender Modifies Defense Dice

Profit!