Does the Magical Star Kanon 100% spinoff fit canonically within the main series?
It's just a spinoff because Kanon is extremely popular in Japan. You can't call it canon (or should I say, kanon), because it is not based off the material from the manga. However, Wakaki has drawn a few bonus chapters based off Magical Star Kanon, but does not hold the premise for the OVA's story.