Where is that valve used?
How often per day/hour/minute (or whatever scale seems appropriate) is the valve opened/shut?

Well, I've been using DC from https for a couple of years (ever since it was made available) now, and I haven't seen a 'mixed content' messages in a long time, other then some members' Forum profile 'Signature' setting where they link to images/sites using http url's

You can instantiate an Excel COM object, and manipulate it completely from there, so I think you'll manage that fine. Lots of resources to be found in 'da google' when you get stuck ;)