Thank you very much for this feedback, it's useful information!
I will admit that I completely forgot that split keyboards existed, otherwise I would've tried buying one and testing it out there!
The melody keys assume that keys 1-8 are a single continuous row similar to piano, so it makes sense why you struggled there.
I was aware of some of the issues outlined even while developing the game for the jam, but at the time I feared I couldn't fix them without breaking the jam rule-set (something for me to revisit for sure).
A couple of examples:
1. I thought about including the keypad for compatibility, but I believed it would've broken the 12 controls rule. But even with that, it may become visually confusing, having the melody keys on screen in one row, but trying to play Nokia keypad as a grid. I suspect it'd require a re-design to work well.
2. Regarding lack of backing track/metronome, I suspect Guitar Hero and Rockband get away with it b/c they play the music in the background while you're jamming. That way you still have the audio cues of the original song to anticipate. Whereas on Nokia, you have only one tone at a time. I suspect a metronome may not help though, b/c you'll have the audio system fight between playing the metronome or the song tones. That'll be worth testing to find out though.