1. Like A Dragon: Pirate Yakuza in Hawaii contains both Battle On The Edge (dome first track) and Power Edition (includes Hornet car and long track with all 3 regular tracks merged into one - this one's also in The Man Who Erased His Name).
2. Holding Start during the "Gentlemen, Start Your Engines" does now change singers in the game (it doesn't in The Man Who Erased His Name). Although it also pops up the menu after a few seconds so you have to press it again and again!
3. The Multiplayer games allows you to run up to 4 instances of the emulator at once, for multiplayer (I tried 2 but didn't set the controls for the others, but assume it just spawns 2 more windows).