Shattered glass. Needed to get replaced
The fix was a breeze. Couldn't have been easier.
During the crash that lead to the glass needing to replaced, the iMac rear case was significantly disfigured. I had mistakenly believed that would make it impossible to get the new glass to fit in the warped frame, but fortunately, the glass just rests on top of the frame and doesn't fit into it.
So, while the frame remained misshapen, and surely dust will get in over time, the glass still fit in place as it should.