Despite not having read the VN (I’ve watched the anime, but it was a while ago I don’t remember much of it), I will answer to Kanon.
Indeed, in the anime, Sayuri discusses Pachelbel’s Kanon in D-Dur with Yuichi while the song is playing in a cafe. In reference to a ‘canon’, she says something to the effect of: “It would be nice if life changed like that: slowly but surely, while seemingly unchanged from day to day.”
I believe this has three meanings. First as literally Sayuri’s feelings which reflect her personality and attitude toward her situation. Second, and more importantly, as a message of the story in its entirety: “It would be nice if life was like that. BUT IT ISN’T”, which they go on to prove vehemently to the audience as each character gets jerked around by one of life’s sudden surprises. Thirdly, finally, and most interestingly to me, is its metaphysical significance. All the routes of Kanon share similar, repeating themes - miracles, music, amnesia - each then changing those slightly, putting its own variation on it. And if you think about it, thats kinda what Sayuri was describing.
There’s only one problem with this interpretation:
That’s not what a ‘canon’ is.
In fact Pachelbel’s Canon isn’t technically even a canon. It’s a chaconne. A canon simply repeats itself (think a version of Row Row Row Your Boat you might have heard), while a chaconne has a repeating base line with a variating harmony.
So why wasn’t the VN named “Chaconne”? Well, first, it was Pachelbel who misnamed it, and as already shown with CLANNAD, Key isn’t known for getting foreign terms right. And besides who would spend money on this relatively new thing called a visual novel, whose name you can’t even pronounce?