Me and my group had a debate due to mix wording on some things, but I dont believe the Apothecary's special abilty that allows him to heal an additional 1d5 wounds effects critical wounds. Am I correct or does it apply to critical wounds?

First Aid can only heal 1 wound of when the wounded is in heavy or critical damage. This means your 1d5 extra wounds is ignored, since you can only heal 1 wound.

There are abilities and wargear which can make a heavily or critical wounded character count as lightly wounded (the character is still heavily or critically wounded, just for that specific first aid or healing attempt it counts as lightly). Allowing normal healing under first aid, including your 1d5 extra healing.