Wow... busy, busy, busy. I'm getting tired just reading this!
I haven't tried Hardibacker, have always used Durock and get good results. While you at the big box, you might want to get one of those carbide tipped scoring tools. It's about $7 and you'll find plenty of uses for it besides scoring backer board.
I swear you can score concrete with this tool, then turn around and cut glass with it.