If you wrote that fic about Space Hisao who is Meiko's son, then I wouldn't expect you to tell the story all the way back to 2008 where Hisao was frozen in carbonite to keep him alive alongside Meiko who later adopted him... Actually, please forget that train of thought.
Anyway. You can write the story you want. Whether or not it will be good depends on a many other factors.
The problem is this here.That's why I'm avoiding using AU terms.
In my opinion it is much harder to write a Divergence fic than it is to write an AU fic. In a Divergence fic you need to be careful to keep everything but the divergence point as you did in the original. You are very constrained in the way secondary characters react. In an AU fic you can basically write whatever you want.
By describing the story as a divergence fic you're creating expectations in your readers that the story won't be able to fulfil.
You say you're avoiding the term AU because AU is viewed negatively by most, but what you are writing is in fact an AU-fic, so you are mislabelling your story.
That doesn't mean AU fics are always bad - many of them are, but there are notable exceptions.
If the story WERE a Divergence fic, THEN you probably should go back and tell everything from the point of divergence to explain how things got from that point to the way there are now - because that's what a divergence fic is all about. You don't need to go to that length for an AU fic.