Jon, you said: "When playing a C in the key of D (C sharp) you play all holes open." I think you meant "When playing a C note on a D whistle..." otherwise it's confusing.
I think you are in a world of dilettanti here. I used to say I specialized in instruments that cost less than $10... and I couldn't play any of them well, either. [grin]
Blessings
Barbara