I recently got an inexpensive HK keychain light with a UV led. It
worked great at first. After a very short amount of time, the
brightness dimmed to a useless level.

Thinking it had cheap 2016 (2) cells to begin with, I replaced them. No
effect. Then I noticed there is no current resistor or even space for
one. The led leads are simply placed above and below the 2 cell stack,
finger pressure makes the contact. So the 2 cells (6Volts) go directly
to the led leads. Does this sound right? Not to me.

I remember reading about reliability problems with UV leds. Is that
still the case?