Fridge Brilliance: Sam leaps into a man named Tom. It is in his very next leap that he remembers that he has an older brother, also named Tom. Perhaps hearing that name repeatedly, combined with the Big Brother Instinct that kicks in on the next leap, helped bring those memories to the surface?
Fridge Horror: Because anything Sam did to change the past would result in a different timeline, the committee reviewing Project Quantum Leap's funding could never possibly be satisfied that he had changed enough; if Al was the only one who noticed the change with Diane, chances are preventing U2 wouldn't have changing the hearing in the slightest. It is only the change that he effects, which results in a fundamentally more sympathetic committee chairperson, that could have saved the day.
In fact, going one step farther: it's reasonable to assume that even if Sam prevented U2, that would've possibly meant a paradox. Who's to say the Project would've even existed in a non-U2 world? Al was betting the farm on a long shot he didn't think through...
