"US is in the lead by more than 5 meters going into final leg. In this case Bolt the 100/200 specialist can not draft behind Merritt for first 300m. No contest US wins."
Do you seriously think Bolt wouldn't close that gap right off? Your scenarios--two relating to race strategy, the third an unlikely switch in actual training strategy that his coach would never follow--hardly apply to Bolt, who'd eat up any 5 m gap in "two shakes", then get all he could out of his superior 200-300 m speed. No way he runs it like a sensible 400m runner: maintaining contact, then battling it out on the homestretch. His only chance would be to establish a strong first 200 +, then try to relax enough to have something left--wisely counting on the energy of the crowd to bring him home. After all, neither he nor anyone else has any clue how he'll do; it's easier to speculate on how he'd do it.