Great, it seems you've got most of it sorted out then.
GIK's gridfusors are a good match for drop ceilings. And just as you posted, it isn't too hard to mount them elsewhere either, if you needed to. And a good price! The Auralex ones freak me out since they're hollow, but I'm not sure if that really affects their performance in a bad way.
Are you located in France? Perhaps you might be able to get some of Jens' Optiffusers for the back wall. They are very light and cost effective as well for a good amount of square footage. The price also includes shipping anywhere in the EU: http://resonatorstockholm.com/optiffuser/