hope the workshop is a success.
one slight correction, perhaps I misinterpreted your statement, the traditional key in east anglia, was only C, as regards single row melodeons,
the fiddle players played in G D and A, although some tuned the whole fiddle down, when playing WITH C ONE ROWS so that they were playing as if they were in D, But were in actual fact in C.
a number of TWO ROW MELODEON players played GD boxes.
Oscar Woods WAS THE MAIN EXPONENT OF THE c one row melodeon, and more latterly Cyril Barber.
I am fairly sure the Bulwers did not play exclusively in C, but mainly g and d, and I am fairly sure both PERCY BROWN and Font Watling played two row gd melodeons.
Reg Reeder played his Hammered dulcimer in C, so C was used but so was G and D.