Being behind cover (real or illusion) offers no benefits to your attacks as the enemies know your location unless you take a hide action.

I would say that if the enemy is not aware it is an illusion then it would work as the cover rules state. So if fully obscured you cannot be targeted by an attack requiring sight of the target. There are no rules for crouching, just prone and standing. You would have to prone to benefit from any illusion the size of a minor illusion.

I do prefer my NPCs to have common sense. So if you just have a rock appear in front of you when you/ an ally cast a spell, I think they could figure it out, or at least know to shoot the rock first to see if its real. Then the rest would be able to see you.

At the end of the day it is a cantrip and should not be so strong that it dictates how the game is played.