Why is it always so difficult to find the details? What kind of cipher was it? How was the key derived from a password?

> “It’s irrefutable,”

Such absolute language is generally reserved for overconfident people who have jumped to conclusions, not people with experience in statistics and analysis. In particular, substitution ciphers on short strings have a significant number of possibilities. This theory is one, but it is VERY far from "irrefutable", even paired with other circumstantial evidence.

So few details

Any reverse linkages, by the Zodiac killer referencing the Black Dahlia killings, are potentially explainable, even the "deathbed" Elizabeth painting, as an interest in a historical murder[er].

What's interesting and not easily explainable if true, however, is the suspicion that the Black Dahlia murderer used a motel that at the time was called the Zodiac Motel. That forward-connection would've taken someone obsessed with solving the Black Dahlia murder, not just interested in the nature of the crime; assuming the theory about the Dahlia murder location is correct, the Zodiac killer would have had to solve the location of the Dahlia murder by himself, and then use it as an in-joke for a later series of unconnected murders.