Concerning magic missile and mirror image, the issue has more to do with whether or not you target one of the images (the illusions) created by mirror image or the caster.
The way I rule it is that in 5E magic missile targets a creature you can see. Can you see the creature with mirror image up? Yes, although you also see duplicates of the creature. Since you can see the target creature, they are a valid target unlike the illusory images of the creature.
Which doesn't pertain to this thread as far as I know.