As for the canon topic, I've always wondered, when talking about canon in the context of D&D (or any fictional franchise), and people mentions "this is/is not canon": is "canon for what?". Because canon is only useful when you trying to "study" something ("I'm reading this series of novels; is this novel relevant/canon to my reading order, or can I skip it?"), or when you're doing a professional or semi-professional research on something (like, writing an article for a wiki).
When you are worldbuilding, canon become less useful. The only use for canon in this context (for me, at least) is when you want to be as faithful to the source material and, as someone mentioned before, for some settings it may be impossible to be really faithful to canon (good luck trying to be faithful to canon in the Forgotten Realms). And as soon as your players interact with the game world, you've already deviated from canon.
I see canon more as a tool for understanding/researching something than a holy scripture.
Is anyone really going to be upset if dragonborn are suddenly in Greyhawk?
I think it depends. If you say to me that dragonborn have always lived in the Flanaess, then yes, I'm going to be upset. Not because I'm a "canon-lawyer", but because I don't like being gaslighted.
But if you say to me that yes, they've always lived in Greyhawk, but in one of the unexplored regions of the world, then I wouldn't have a problem with it. It would be a new development for the world, and I would like to see how they introduce it into the established lore.