I seriously believe that the unused layout is, in fact, the single-cart multiplayer level. The GBA supports getting multiboot code downloaded over the link cable into RAM to be played; one of the main uses for this is to allow up to 4 players play a multiplayer game, but with only one copy of the game used. This is present in Advance 1 and 2 with a Capture the Flag-esque game mode, in which players run through a small zone collecting the most of something (I forget what the goal was in Advance 1, but in Advance 2 it was Chao). The reason it's looking garbled is most likely because the graphics are compressed, as the entire mutliboot binary has to be shoved down the link cable to each player, so they had to do SOMETHING to make the download go quicker, and the GBA BIOS already has a LZ77 decompression routine for the game to use. I wonder if a modern emulator like mGBA can capture these multiboot downloads?