Nabiki isn't really changing her personal timeline unless she actually
comes from the one where Ranma died right after she met him, and I'd
assume she has reasons for not changing the death of her mother, 
otherwise she'd do it.  Most likely that she was only allowed to 
make a small initial change and couldn't find one that would prevent her 
mother's death.  Really though...this story could have been written 
almost exactly the same by using a living Mrs. Tendo rather than Nodoka.
Perhaps the biggest mystery is how Nodoka could find Genma to have that
phone conversation, though.