A composer writes a piece in a particular way, but once it has escaped from captivity they have little control over what happens to it. Indeed, anyone composing and performing in a traditional idiom should not be surprised when people start to play it in a different way.

As for playing in the original key, people will adapt the key, and even the tune, to suit their instrument. I've never got the idea of different keys having different moods, does this idea still apply when everyone is using equal temperament? Certainly different keys have different characteristics on particular instruments, and that may have had more to do with the composer's original choice of key than any theoretical ideas of moods. However that cannot necessarily be transferred to another instrument, and the characteristic the composer intended might be brought out better by playing in a different key. Or should tunes only be played on the instrument they were composed for?

Should an old tune only be played at the concert pitch current at the time? Is it wrong to play an old tune in A=440?

The point about folk music is that it is susceptible to change. "The Moving Cloud" is a folk tune, even thought the composer is known, because different versions have developed and it is now out of the composer's control. "Masters of War" isn't a folk song, because Mr Dylan's lawyers will sue the arse off you if you attempt to change it.