Roddenberry flat out said TAS wasn't canon
Not quite.
Richard Arnold penned some official memos and "ST Communicator" columns that said that the
ST Office didn't consider TAS to be canon, but that was
after Filmation was wound down and sold off - and during a time when DC Fontana, the showrunner of TAS on Gene's behalf, was suing him for co-creatorship of TNG.
During the 70s, there was no such reaction to TAS - and I once read an interview in which GR joked that he once tried to coerce Majel Barrett into doing a live-action M'Ress dual role cameo in ST:TMP.
I even think I read somewhere he wanted to say the Motion Picture wasn't either.
No, he was referring only to aspects of his novelization that did not appear onscreen.
He was also said to have considered "parts of ST V to be 'apocryphal'." He went into a coma before making a similar determination on ST VI, although he did comment negatively on its script.
Jeri Taylor has flat out implied her Voyager novels are canon since she was one of the top people in that franchise and Janeway was pretty much her baby.
But the rest of the production team began
ignoring aspects of them from the moment she left. And she was fine with that.
how is the casual fan supposed to take what is and isn't canon seriously?
Because the canon debate really only started when Richard Arnold attempted to find a measurable/predictable stance to quell those more rabid fans who used to berate Roddenberry for not exploiting aspects of ST created by others without his consultation: ie. challenging him at conventions, and in letters, as to why Romulans weren't being called "Rihannsu" (as suggested in novels by Diane Duane), or why Kirk didn't just blast the Klingons with dreadnought battle cruisers (from the Franz Joseph designs and the war-heavy "Starfleet Battles" RPGs).
Especially when so many "non canon" sources have produced better quality work.
Which is debatable by other fans - and is sure to make the creators bristle.