afaik, you need a GBA everdrive for GBA games. I haven't found a path for GBA game operation through the NDS cart that doesn't involve glitchy software emulation, rather than the native GBA playing ability
haha! second screen in the battery compartment! love it!
FYI, you've used ribbon cable for the DPAD and buttons but you could easily use thinner wire like 80-pin IDE cable as they are only signals not power. would make the cable management easier as those wires can be a pain to deal with...