[bitwize]

The bank switching mechanisms in all commercially released games can be enumerated and emulated by the 2600+, but I think they just haven't gotten around to updating the firmware. Many bank-switched commercial games still don't work afaik.

Flashcarts are more of a crapshoot. They use unknown bank switching mechanisms and Atari does not want to support their use.

> In the case of the imaginary cartridge with the UI...

Not so imaginary! This is what the pack-in cart that comes with the 2600+ does, there are DIP switches for game selection on the cart itself. I don't know of any flashcarts that do even this, let alone something more sophisticated with buttons and an LCD readout like a Gotek drive emulator or something. Would be nice though...