Regarding the variability of pitch, the table about a quarter of the way down this Dolmetsch music theory page gives some idea how it has changed over the years. Generally orchestral pitch has drifted up; Mozart's E flat is only a little above our D. But British military bands generally played almost a semitone above A440, and you have to be very careful when buying old simple flutes that they are not in this pitch. Unless of course you want a flute to play along with the whizkids who tune their fiddles to E flat. I have a lovely old wooden Ruddall Carte 1867 system flute, but sadly about half a semitone high in tuning.

I've often wondered, in the light of this, how keys can be said to have individual "characters".