1) What exactly "is" Animal Welfare?
It's like art or--if your prefer--pornography. People know it when they see it. Having more precise definitions in more precise settings is fine, but don't expect the social issue to conform to your chosen definition. Reality creates semantics, not the other way around. And in most cases we can get along just fine addressing issues without paragraph long quizzling "defintions', the creation of which is often used as a substitute for real action.
2) Why focus on animals when so many people in the world are suffering?
I'll focus on what I want to focus on, and you can focus on what you want to focus on. But, frankly, a compassionate and just society tends to extend consideration and protection to all, to the best of its ability and resources. I am just one of those resources. So are you.
3) But I see that you [eat meat, leave your dog at home while you go to work, insert something else questionable here], so how dare you tell me what to do?
I try and tell people about the consequences of their choices and actions, what they do is then up to them. Like them I am not perfect, I am sometimes even hypocritical. Feel free to call me on it, but try and keep it somewhat on topic and be sure to do the same to yourself. Then we might both learn something and make some better choices in the future. Nobody is perfect, which is why everybody has room for improvement.