> modded 2DS using the DSi emulator

The DSi and GBA modes on 3DS aren't emulation, there's an actual Arm7 and GBA/NDS IP blocks in the 3DS. For the parts that do require software intervention (DSi RTC, input remapping, etc.) it's more-or-less hardware virtualization.

Anyway, good job OP.

I did a double-take at

    > Executes one line of script per frame (~60 lines/sec).

Makes the "runs at 60FPS" aspect of the engine feel a lot less relevant. At this speed, anything more complex than Pong would be a struggle. Even a CHIP-8 interpreter is usually expected handle a dozen or so comparably expressive instructions per frame.

A DS set to Auto mode will boot to the cartridge (and you can reflash the firmware to skip the health and safety screen). From there the OS is replaced with whatever is on the cart. A flashcart with the right shell will boot right into whatever app you want (and you can soft reset the console with a key combination to switch apps).

3DSes require a little more work and have a longer boot chain, but it's been thoroughly broken all the way to the bootstrapping process so you can use whichever firmware version and whatever patches you like with enough effort.