for me, what is regarded as canon depends on what source comes first. for example, if certain avengers come together to fight a certain enemy and events unfold in a certain way in the comics but in the movie it differs as in say they use different characters to fight or the outcome is different, then the movie cannot be considered canon as the comic came first. the film is now a reboot or remake in someone elses image.
as for the novels. its hard to say really but id say novels with major differences cant be considered canon like the AC novel imo isn't canon. Daniels is aware of the David/ Walter switch at the end but in the novel she isn't. the movie came first and this is a major story arc for the sequel so for me the novel isn't canon. plus there are 3 face huggers brought onboard by David not the 2 we see in the film. such blatant and obvious differences are either stupidity or its done to make it clear the novel isn't canon. or did AFD have access to an early draft of the script and there were indeed 3 face huggers and Daniels was unaware it was David and the novel was released before he got to make the appropriate changes? if this is the case then Fox shoudlv known better than to allow its release before the changes but it still doesn't affect its outcome. I don't consider the novel canon as the novel contradicts the movie too much and in too many vital and obvious places
the comics and novels (non movie related) CAN be considered canon by individuals as they happen in their own timeline usually and there isn't much to discredit them as we haven't seen a movie that shows a different set of events that contradict them. river of pain is a prime example. it shows us what happens to Newts parents inside the derelict but as the movie never covers this and no one has said otherwise, surely those events can be considered canon? the only thing that can discredit River of pain is where RS takes the prequels to tie into Alien. if he shows that David for example steals the vessal we all know as the derelict, loads it with eggs and crashes/ lands on LV426, then the events of ROP involving Newts parents within the derelict is discredited and no longer canon as I assume the movie will take priority canon wise. personally I love ROP and would rather have its suggested turn of events to be the case but that's just me.
