maybe you are trying to wind us all up here, but my answers are, and I believe all answers will be different, as we are all individuals:
1) for me, I like irish Music so I play it, I also enjoy my own heritage which is a rich tapestry of a lot of things.... born in Africa, to a Yorkshire man, descended from a Scots writer by the name of Burns, and a Geordie Mum whose father was of Irish Descent, who fought at the Passchendale in WWI. I enjoy and partake in the customs, and history and songs surrounding my heritage.
2) I personally have never played the blues, but love to sing it! I am white, female and again dont see that it makes much difference, as long as it is played/sung and enjoyed. I do not ever pretend that I am ever anything other than what I am.
3) Repeat above, I do not want to be anyone other than who I am, I am proud of my uniqueness. In partaking of other ethnicities music and song, I do not feel that I have to pretend to be them.
4) Kletzmer music is wonderful! as to whether it has been invaded..........can you do that? Kletzmer music is Kletzmer music, whoever plays it, it doesnt mean it is no longer such!
5) I would guess that these classical musicians from the far and middle east do as you suggest because they either enjoy that kind of music or it is more profitable. As for Indian Instruments and western players, I don't know, but there are some amazing sounding instruments out there.
Enjoy Music of every sort wherever and whenever you can, with such a rich tapestry for us all to share, we should be grateful that music is shared and not kept secret and sacred to its source - should that be the case, the world would then I feel have been a poorer place.
Cheers, Lucy