[bpape]

Your call on the speakers. More, smaller speakers placed better will give you very good coverage and much less boom. Any shot to at least get those out of the corners?

The pillars would be square most likely to fit in with the look of the room if possible. Do what you can. Probably something like 12x12" is about all you're going to get away with. If you can do more, that's great. You could potentially also do up around the wall/ceiling perimeter to stay out of the way and get a little more coverage.

I wouldn't hang a bunch of stuff on them. They'll be basically frame and 703 with cloth covering.

Cloud is effectively a hard surface. Make it as big as you can manage and still have nice headroom. Might consider not being convex but just flat - but dropping down by the stage and then continuing up at an angle toward the rear of the room. It can be a simple frame with drywall if you want. Insulationis nothing fancy. If you use steel studs, there's not a flammable item in it at all.

Again, I'd seriously consider a couple more smaller pairs of speakers to help coverage without pushing the mains as Frank suggested.

Bryan