There isn't really an officially-sanctioned canon.
The Walt Disney Company at large (and most publishers) tend to treat the Donald Duck and Mickey Mouse universe as a franchise aimed mostly at kids, and therefore, one that doesn't warrant thinking long and hard about "details" like that. As far as they are concerned, writers can do anything as long as they don't characterize long-running
characters in too jarring a fashion.
However, individual people who create or edit official content can have their own "head-canon", which they may abide by for their own stories or when editing others'. For instance, Don Rosa is well-known for defining the "Barks-Rosa Universe", a restrictive continuity where only his stories and Carl Barks's really took place, the rest being null and void. Egmont Publishing also has a set of guidelines that tell the writers to stick to a number of "facts"; such as the idea that Ludwig von Drake's existence should be ignored or that Magica De Spell is a sorceress, not a witch.
Thus, on this Wiki, we have been forced to create our own policies for what is canon and what is not. They aren't any truer than anything else you might make up, but they are thorough and self-consistent.