The piano keyboard is familiar to us all whether the piano is our primary instrument or not.
We have all been taught where middle C is - or at least how to find a C somewhere on the piano and we can guess which one is the most middle. So, the keyboard interface was started with a pretty obvious understanding.
You need to be able to play a single key and not have any other key next to it play. Simple enough. I wanted more.
When learning to play the piano, I was taught that chromatic scales were written in sharps when ascending and flats when descending. So I incorporated this in the relationship between the piano and the notation. Glissandos were the answer. When the finger was dragged up the keyboard, the notes would be written in sharps. When dragging down, they would be written in flats.
The problem of how to notate with a specific accidental by using the keyboard had been solved.
While dealing with the different fingering charts, choosing the pitch with the piano or with notation was relatively easy. A fingering was for pitches. It didn't matter if it was written in sharps or flats, the fingering was for that pitch. But notes are written with either a sharp or a flat. You can't require the user to respell the pitch to your way just to get the fingerings.