but doesn't a fictional element (usually a character) from one show, appearing in another one, always prove that the two shows are in the same verse?
No. It doesn't prove anything at all.
Look at all this Tommy Westphall crap, for example. Taking the final scene of St. Elsewhere
on its face, and accepting that the mere presence of a character is evidence of crossover, about 90% of all TV series in existence would be linked, through that one character. Just sharing a character is not enough to prove a link. You've got to have regular, scheduled crossovers. Like L&O and Homicide.
You cant have the case of Law&Order in one episode watch (or talk about watching) the X-Files and then in the next one, meet Mulder & Scully?
No, that wouldn't make any sense.