I'm not sure what part of date you're having problem with, if you mean what to do exactly after you asked her out, you should change into the suit (drawer in your room) and go to the infirmary at 21:00.
Save files location was changed in one of the recent updates. More info here on where the old and new save locations are:
Nope, quick saves have an individual slot, it's slot number 49. Autosaves also have individual slot - slot number 50.
She hasn't got any more content other than the replayable alley scene. So for now it doesn't affect anything. If she ever gets more content - maybe, but it's unlikely that it will affect content with main girls.
I got what you meant. Maybe. After all knowing the name of boutique salesgirl did have an impact, although very small. But as far as I know it's not even when she gets developed, but more likely if she gets developed. I understand that she hasn't got any update in a long time and that most people more interested in main girls development rather than her.
