If you sink the cushions into a 1" high 'fence' they shouldn't go far, but you could also cut up sheets of that flat lacy looking rubber carpet underlay to put underneath and hold them. I'd be very careful building in sofas - be very sure of the room layout otherwise, because the urge to move things around in future could be severely limited by them! Re sofa pillows - don't forget to get the angle right on wedge pillows for backs - uprights are uncomfortable, but if you use loose pillows make sure there's still room for you and that they're not lumpy in the middle.

We used to have problems with cushions sliding forward on our rattan loveseat. It was basically just a rattan and wood frame with plastic between frame and cushions. I removed the cheap plastic and replaced it with strong naugahyde, (remember that word?) rough fabric/backing side up. Stapled it firmly all around frame and no more sliding. Still fine 30 years later. Sandy