mega-hugs to you as you try to untangle all of this.  I think the temperament/diagnosis issue  may be one of the most interesting questions in psychology right now - at what point is an active kid someone with ADD, at what point is a shy, quiet kid someone with AS?  On the one hand, I work in this field and see the benefits of diagnosis.  OTOH, I also see kids for whom no diagnosis really fits, and whose parents are struggling as you are.  

FWIW, the labels/diagnoses are mostly just behavioral descriptions  - behaviors that professionals have grouped together.  The grouping of behaviors into a diagnosis helps alot with treatment or education, but it's not the full answer.  Richard Roy Grinker's "Unstrange Minds" does a great job of teasing this part out.

Two things you said struck me, though.  First off, I think gender was revealed somewhere in the thread.  There's no question that girls with AS present much differently than boys.  There was a great "Primetime" show on ABC that focussed on this - many women didn't get diagnosed until adulthood, and still don't look "typically" autistic.  ((FWIW, I tried to find a link to this story for you, but I only found it as a DVD on Amazon.  Maybe it will re-run on TV at some point))

Also, the genetic underpinnings of autism mean that traits of autism and aspergers run in families.  If you have a brother with autism, it does mean that other people in the family may have some milder traits - language issues, social discomfort, etc.  These may not rise to the level of diagnosis, but they are still there.  One coworker, whose son has Aspergers, told me about a recent family reunion where "everyone looks and sounds like John!"

Don't know if any of this helps with your particular DC, but I think either way (temperament or diagnosable) the approach would be similar - teaching skills that make DC's life easier and happier, and adapting to traits that don't change.