What happens at the end of this episode? I missed
it.

He helps her pass the bar. Al is in a Senate hearing, listening to the Senator about to shut down Quantum Leap when all of a sudden the old man Senator is replaced by the wife as the Senator in charge, who announces the continued funding of the project.

Cool, thanks. Does Al see the change in the timeline?

Yes he does, and he's shocked.

At the end when the lady Senator says that she seems to recall meeting someone named "Beckett", Al should tell her to remember her honeymoon, as proof that QL works.

The senator at the end---who I assume was portrayed by the same actress who competantly played Diane McBride throughout the rest of the episode---was completely unconvincing. Granted, she was given implausible lines (trying to gloss over in two seconds the significant challenge of why the governement should shell out millions of dollars for a stalled project whose lead has vanished) and a lame gag at the end, but the delivery was like she was reading unrehearsed lines off a teleprompter. It was jarring.

Al's plan seemed very risky. He wanted to revise a major geopolitical event, under the assumption that Sam could hasten the end of the Cold War. But the West won the Cold War. Sure, it took a long time and had substantial costs, but why didn't anyone worry that changing history in such a major way might result in a less-desireable outcome? Could Ziggy really extrapolate assured victory, after the timeline that they know assures victory in thirty years has been disrupted?

I found the business with the gun confusing. Near the end, Sam insists that he needs the train's gun. I don't recall that gun being established earlier. In his next scene alone with Diane, he seems confident that Roget is no longer a problem. Why did he need the gun, then? And moments later Roget disarms him, and disposes of the gun. What purpose did the gun serve to this story?