It is important to remember that lies are easily detected.
... the fantasy of having proof of a 2nd shooter provides a simple key that redefines the event in a way that unlocks a different indexical chain of legally necessary cause-and-effect.
Dramatists like to imagine lies being raised up in scenarios to be tried and tested, as in a crucible, under the highest standards of skepticism. The dramatic idea is to strip away all extraneous details and confounding incentives until the verity of the characterization by some character reveals a profound character choice.
It's kind of interesting that people don't labor over discovering lies, but over proving lies. It doesn't take much time or effort to detect lies. Things seem off. It happens pretty instantly. The exact nature of the lie takes a bit of leg-work, but that's it.
A lot of the enjoyment of the sleuthing-style conspiracy theories comes from accessing a similar space to lying: "I have singular access to an ephemeral event whose existence is unquestioned yet whose character will directly and unalterably determine the choices of an untold number of people."
Something like the JFK assassination involves an event that is treated like an index for a bunch of administrative shuffling (POTUS dies -> new POTUS) and the fantasy of having proof of a 2nd shooter...