1)polyhedral sound like a good idea, I'd washout the wing tips too
2)A plane that big without ailerons, not to good, weight shoud not be all that much of an issue, for what you gain in control. How about spoilerons???
I would even add flaps. lol
3)just suport the spars like you would for symetrical wing. Design your spars so that they hold your ribs in position. The truss idea sound like a real good way to start!
4)Built up balsa rib, lots of work, but would be lighter and stronger than balsa sheet.
Someone I know has a built up rib for a full size Jenney hanging in their kitchen. you would be suprised at how strong and light it is.
I think foam would flex too much to be useful on something that size.
Carbon fiber on the other had is one of the best tips you have gotten so far, you would be amaside at how much you can stiffen a wing with it.
Check out:
http://www.airfieldmodels.com
He has a lot of good design and building tips!
Best of luck have fun with it!
Paul