Question: What classic song are Joan and her friends listening to when the Russian and Frank Castle are fighting in Castle's room?
Question: In the extended version, when Frank confronts Jimmy about his betrayal, why did he make Jimmy commit suicide instead of killing Jimmy himself?
Answer: Jimmy is drowning in massive gambling debt and has had to resort to selling everything of value he owns as well as steal money from evidence lockers to help pay his debt down. He has nothing to live for and is merely going through the motions of life, and coupled with his guilt over selling out Frank and getting his family killed, Frank concludes Jimmy needs to take his fate in his own hands.
Question: Would Castle's life have really been saved by jumping into the bathtub before the grenade went off?
Answer: In Season 9, Episode 20 of "Mythbusters," Adam and Jamie tested whether a person could survive a "toilet bomb" (recreating a Lethal Weapon 2 scene) by jumping into a cast iron bathtub and being covered with a bomb blanket. They used 1 kg of C-4 explosives that created a blast with a peak lethal pressure of 180 psi outside the tub. The pressure inside the tub was recorded as a survivable 8 psi, though with probable hearing damage. From what I read, a grenade has a much lesser psi force than what the C-4 explosion produced. Depending on the circumstances, it seems plausible that a person could survive the force and shrapnel while inside the tub.
Answer: It's plausible but highly unlikely. Assuming it's an old metal bathtub (which seems to be the case), it's possible it might have deflected enough of the percussive shock and shrapnel to save him, but unlikely that it'd stop everything. It's one of those things where it probably wouldn't work 90% of the time... but there's that 1 out of 10 chance it could possibly work if he got really lucky and no big pieces of shrapnel came his way. (Plus, stranger things have happened in real life).
Question: Why doesn't Stan Lee make a cameo in this movie, the second Punisher film, or in both Ghost Rider movies?
Answer: Simple - Stan Lee never created these characters. They were created by the people who worked on the comics. The only Marvel films that he appears in are ones he had a hand in creating.
Question: Why didn't they have Jigsaw as the villain since he's one of The Punisher's better known enemies?
Answer: They were actually planning on using Jigsaw as the villain in the sequel until Thomas Jane and director Jonathan Hensleigh exited the project and the studio rebooted the character with Punisher: War Zone. They probably just wanted to show the Punisher's origin first, which would allow him to be fully established by the time a sequel came around, and they would have more to screen time to establish Jigsaw.
Answer: The song is "La donna é mobile" from the opera Rigoletto.