For the manga, unless the author states otherwise, if it's not in the books, it's not canon. And if Toriyama says something that didn't happen in the books, it's not canon, unless he wrote it. When it comes to the anime, it's not his canon, it's Toei's and they never said what's official, so you can accept whatever you want for their cannon unless they say otherwise. The more money it made, the more we got! It seems only if people like something will they include it. I love History of Trunks, but it's non canon in Toriyama's books, but I like it, so I incorporate it in my view of the story. The reason why I think, people were accepting Battle of Gods as a continuation of the manga story is because of this press release statement: "[a] new story in the official history of Dragon Ball is born, neither a spin-off nor a side-story, one that can be enjoyed by both children and parents, manga fans and anime fans." All the amazing changes that Batman TAS made were not part of the comics "canon", until they were written in. However, I used everything I knew from that show as a basis for my reading, unless it contradicted the story I was reading. Maybe I'm getting confused on what canon means vs continuity, because I never heard of "canon" before getting into DBZ. All I know is I love Dragonball!
I just checked and I didn't see Cooler at the end of the movie. The episode of Bardock took place thousands of years ago, being that Vegeta said the last Super Saiyan appeared a thousand years ago in the Freeza arc, and we know there were multiple Super Saiyans, being that there was a Super Saiyan God, and Bardock proceeded him. There had to be a lot of Super Saiyans, being that they were able to over power a Super Saiyan God. Then again, he was pure of heart, so he probably didn't go all out on them, but still they must have combined all their energy, and killed him like Goku took out Broly.