[Josuah]

Decouple the speakers from the stands using reasonably-sized rubber bumpers or some other such. Blue tack will not do enough. Also make sure the stands are massive. If metal, flick them and make sure they don't vibrate or fill them with mass to prevent vibrations. If wood, you really need to make sure you've decoupled well enough.

Stands that are vibrating with acoustical energy have a very audible negative effect on the sound.

As for questions #1 and #3, I'd post on Ascend's forums or ask them directly for the best answer. I don't recall if the Sierra 1 center's tweeter has a particular orientation associated with its implementation.