Is Kanon's song really only filled with Kanon's love for Keima?
Nothing mention that goddess power source is only come from love toward opposite sex, I think it's also include family love, friend love, animal love, and other kind of loves. I do not think taking what Apollo said as plain wrong and the love filled in the song is Kanon's love for Keima is a good thing to do. Maybe, yes, it's included, but also her love for her fans.
Goddess and Runaway spirit although different have some similarities. I think if we use Runaway spirit growth level as comparison, Diana will be level 3 (can talk with her host, can use her own power, can take over her host's body and have a halo) while Apollo still level 1 (only able to talk).
In
his blog, Wakaki said that he already thought of goddess concept from before it get serialized. I think it's only his lack of skill that makes readers thought otherwise.