[QUESTION]

A shoulder pad on the left shoulder, a breastplate on the right. Why can't there be something similar on the chest?

Maybe. But you're writing as if you're grasping at straws just to defend your point. But the point isn't that, but rather that you used this picture as an example of a girl in a breastplate, but no one sees a breastplate on it, it doesn't matter if it's on the second breast or not, we'll never know, so why even use this particular picture as an example and write that there might be a breastplate? This also applies to the next part of your post.

armor and bikini

So, armor in one place + bikini in another = breastplate, great logic.