#1 So? You still play it like one.
#2 Just like a bass.
#3 You can probably flip it over. Many games incorporate this. Brain Age, Hotel Dusk, etc.
#4 Depending on the quality they are shooting for, they can fit as many songs on as they like. They could even sell the stand-alone cartridge if they want. If they were smart, they would have the game data on the GBA slot, and the song data on the DS slot. They probably wont do that though.
#5 As with the rumble pack, I wouldn't be surprised if you were left out, or had to order special. Even if nothing was available, I am sure a 3rd party accessory would show up.