
I don't think the questions you raised are relevant from my point of view. If that person was forgotten to add or did not appear in that episode, those people will have their answers very soon when they are watching that episode, cause I am sure that people don't care about it when they are not watching that series.
It worries me more if someone who is not watching a specific series starts adding cast, cause what source do they use and is that source correct/reliable (F.E. IMDB is not reliable at all)?