Sorry I am so late to this conversation, but I do love Kabuto, so I can't keep silent forever.
I think God Speed Love really is the prologue to the series. HOWEVER! The ending of the movie should not be interpreted literally, but rather it should give us a hint of how it actually happened. In the series, if I remember correctly, Tendou never saw who gave him the belt. I think that's how it should have been. Tendou came back in time to make a, hopefully, better future than what he saw, but his kid self never actually saw it because his vision was blocked. I don't think future Tendou actually un-henshin'ed when he gave his younger self the belt. After all, Hiyori had a perfect view of him, and he did kind of look like that fairy or whatever from her drawing when he appeared before them in Hyper Mode.
Now, from what I understand, people are bringing up facts that imply that it could have been someone else who gave the belt to Kabuto. It's been a while since I saw Kabuto and probably need to watch it again (Maybe this time, I won't get confused by what's going on so much). But I think those facts are only there in an attempt to make the series stand on its own and make the movie a little more optional, even though the series is so confusing it practically needs the movie anyways.
Even the movie can't be called a movie (hope you understand what I meant).
No, I don't understand. Please, do elaborate on what you mean. Because as far as I can see, it is a movie. It's longer than an episode, it was released in theaters first, it sort of ties into the series in a confusing way. I don't see how it can't be called a movie.