Yes, it can become part of one's culture to appreciate another's culture

It's possible, sure, but you'd have to do a study. As I mentioned, there are plenty of ways to interpret it.

I thought by adding "or whatever it's called" would make it clear that I'm aware that the theory is a racist and antijewish lie, but I guess that was not enough. How should I have worded that differently?

Ah, OK! I think "if white people were defensive of their culture" is what threw me.

What would it mean to take the time and effort to learn to play the Erhu, understand its history and context, but somehow not show real respect for it?

To begin with, there can always be some jerk out there who gives you a hard time no matter what, or who has had so many bad experiences with ppl thoughtlessly appropriating culture that their mind is just closed and they react badly. You'd just have to defend it and let reasonable ppl see that that person is wrong to call you out. That aside, I think showing respect means that if an instrument is sacred for some reason (I have no idea if the Erhu is), you don't play it in a profane or silly way. Outside of that, using an instrument as like a way to make fun of the culture would be bad (e.g. playing it whenever a stereotyped character appears on screen). Just my two cents.