There is no "conspiracy." Apple lost a prototype device. There are prototype devices all over eBay. Leaks happen to every company. You really ARE giving Apple too much credit- it's a brand, with buildings, and employees who occasionally f-up. Case closed? Good.
Two seams are critical- one is for symmetry. The top doesn't have a pairing symmetrical seam because it isn't as important as the side seams. The top is hardly symmetrical to begin with, considering the headphone port and sleep/wake button. The seam doesn't detract from any nonexistent symmetry that you claim was there to begin with. The necessary seam on the bottom of the phone, however, is isolated from other components, and as such, it would be disconcerting to have only one seam there, when the asymmetry is so obvious.