To anyone, correct me if i'm wrong but this is what I think is right.
1. Instead of that sentence, it wold be more precise if it were to be
a) "I've been eating roast duck ever since I lived in China" or
b) "I started eating roast duck ever since I lived/stayed in China."
2. With the sentence I put, the meaning would have been that
a) i've been eating roast duck from back when I stayed in China until today. I grew up in China and my actions started from then.
b) i've been eating roast duck ever since I started living in China. the action only started from when I was staying in China.
3. i'm not too certain of what you mean by that but I assume that you're asking how A should ask B if B were to answer "yes/no. I've been eating roast duck ever since I lived in China"
This is how A should ask : Did you start eating roast duck ever since you came back from China?
Again, there are many ways to ask, it depends on the person's point of view in how to question is interpreted and how they wish to answer the question.
i'm not that great in explaining but I hope it helped a little ! Please correct me if i'm wrong :) happy learning !