#1 - Nice. He sings "A British tar is a soaring soul ..." and "I am the monarch of the sea, the ruler of the Queen's Nav-ee ..." from Gilbert & Sullivan's HMS Pinafore. I thought that was a nice touch of color for action movie.
#2 - Maybe easy, but Samuel L. wasn't well known at the time of the movie. I was surprised to recognize him when I saw the scene again on TV many years later.
#3 - Hint: they all reprised roles made famous by the same actor. In one the character was renamed, but the movie was a straight remake and it was clearly the same character.
OK, you've knocked down all my (I thought) impossibly tough trivia. Just some cleanup:
#2 was Coming To America #3 Tom Hanks played the ringleader in the remake of The Ladykillers. originally played by Alec Guinness. Gary Oldman played George Smiley in the remake of Tinker, Tailor, Solider, Spy played by Alec Guinness in the original miniseries, and also Smiley's People. Obiwan Kenobi was played by Ewan McGregor in the Star Wars prequels, and by Alec Guinness in the Star Wars actuals.
I've got one more really tough one. I doubt anyone will get it.
#4 Movie whose title is the true name of the actress playing the title character, though the actress is actually much better known by her stage name.
The actress born as Diane Hall, and known to her friends as "Annie", changed her surname to Keaton (her mother's maiden name) early in her acting career to avoid confusion with another Diane Hall.
Also interesting (to me at least): Michael Keaton was born Michael Douglas, but borrowed Diane Keaton's assumed surname to avoid confusion with the Michael Douglas, son of Kirk Douglas. Although that other Michael apparently was really born with his name, his father was born Issur Danielovitch and changed it to avoid confusion as a guy with a funny ethnic name. (Times were different then.)
Here are some real names from Hollywood's Golden Age. What are their stage names?
Norma Jeane Baker (or possibly Mortenson)
Now I'm tapped out. I'll shut up and let the thread die ... unless anyone else want to contribute!
Marion Morrison - John Wayne. I'm 99% sure his real first name was Marion, but not sure at all about his last
Archibald Leach - Carry Grant
Bernard Schwartz -
Norma Jeane Baker (or possibly Mortenson) - Marilyn Monroe.
Bernard Schwartz I had to google. It was familiar, I just could quite pull it out of my brain. Then I smacked my forehead when I saw the images that popped up.