From the above, the attacker targets the creature, not any of the images. The spell itself is the only thing that can switch the target to an image; the attacker cannot do so.
The spell contradicts itself with its horrible wording. It specifies that it's impossible to tell which of the images is real, therefore it's impossible to target the caster.
Correct. It never misses what the caster aims at. If the caster aims at an image, it will unerringly hit that image.
It doesn't detect mage armor. It just hits despite it. Being a force attack, it can impact armor and still do damage. It has no need to zip around to some sort of opening and hit there like you suggest. It will go around a corner or cover, but that's it. It also has no ability to tell the difference between one person and another. That's what the caster is for. The caster has seen his target and aims at it. The spell then avoids all but that target. It's not an intelligent thing.
All this time we thought you were arguing about the magic missile spell described in the 5E PHB! It's now clear that you are talking about the version you made up in your head!
The spell does say that it's impossible to track which image is real, but it does not say that this makes the creature impossible to target!
There is a difference. In one version, the correct version, you can, at times, tell which is the real image. You can clearly tell which the real image is if you attack and hit the real image. Though you would then quickly lose track as the images continue shifting.
This is you enunciating a "capricious and arbitrary" set of rules to rationalize the spell's behavior while allowing it to function mechanically as you say it should.
No. this is me explaining to you the spell as it is written. You select a target. The spell cannot discern the difference between an elf, a dwarf, a fighter or a rock. It merely goes to the target you selected.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.