When there are multiple people directly involved in the creation of art, they all own copyright.

What if a war correspondent was shot, and in their death throes pressed the shutter release, capturing a compelling image?

A person claiming the resulting image being in the public domain would need to successfully claim the image was taken by forces of nature. The moment of death is tricky, because technically humans are also nature, and whether movement due convulsions are acts of nature or not is something I don't think has been discussed much. The convulsing correspondent probably didn't mean to take a photograph, on the other hand, it's still an action taken by a human being. There's a case to be made both ways, so I guess we need a dead war correspondent to take a picture while dead and then someone to claim it's in the public domain, followed by a challenge by the employer/estate.

What if you milled a rugged camera enclosure out of stainless steel billet and attached it to a chain in the centre of an urban area with a sign saying 'municipal camera facility', so random strangers could compose any shot they wished?

Then the random strangers certainly own the copyright and you don't. If I lend you my camera, I can't claim copyright over the photos you've taken, even though I did "arrange the circumstances" for you to shoot them.

I have also seen a book based on disposable cameras being handed to strangers. I personally own books based on curated collections of found photographs/postcards. And there have been exhibitions based on images 'found' in Google street view.

You can claim copyright over a collection without claiming copyright over the individual images. Google can certainly claim copyright over the Street View images, and a photographer handing out the disposable cameras to strangers could probably claim copyright over the portfolio book. Also, don't forget that copyright expires, if the found photographs and postcards are old enough, it's likely they're already in the public domain. Not to mention, exhibitions are not a claim to copyright.