More about the weird behavior of ReferenceWritableKeyPath with an Optional property

Question

(I say "more" in my title because this question is an appendix to my How does Swift ReferenceWritableKeyPath work with an Optional property?.)

In my code, self.iv is an outlet property:

@IBOutlet weak var iv: UIImageView!

Now, will this compile?

    let im = UIImage()
    let kp = \UIImageView.image
    self.iv[keyPath:kp] = im // error

No! We are told that something here must be unwrapped, although it is not entirely obvious what it is:

Value of optional type 'UIImage?' must be unwrapped to a value of type 'UIImage'

But now watch this. Instead of an outlet, I'll just make an image view:

    let im = UIImage()
    let kp = \UIImageView.image
    let iv = UIImageView()
    iv[keyPath:kp] = im

That compiles! Why? What semantic / syntactic difference is there between a UIImageView property and a UIImageView local? Is it that the property is an implicitly unwrapped Optional?

To test that idea, let's "propagate" the unwrapping of self.iv explicitly through a local:

    let im = UIImage()
    let kp = \UIImageView.image
    let iv = self.iv!
    iv[keyPath:kp] = im

That compiles too! But we must use a separate local; merely unwrapping self.iv directly is not sufficient:

    let im = UIImage()
    let kp = \UIImageView.image
    self.iv![keyPath:kp] = im // error

I am not understanding this at all. It goes against my standard beliefs about what an implicitly unwrapped Optional is, and about how a non-Optional may always be assigned to an Optional wrapping the same type. Something about ReferenceWritableKeyPath seems to work oddly with Optional properties but I can't put my finger on it.

So my question is (sorry, maybe a bit vague): can someone justify all these results and make them seem rational and predictable? What are the rules here?

NOTE: Could be the same as https://bugs.swift.org/browse/SR-11184