Yes I think a single country, ruling other regions as supplicants to itself, is a bad thing. In theory I guess you could say it doesn't inherently mean a bad thing, but history suggests that very few actual examples of it, actually had the other people's best interests in mind.
Yes, but the reality is that the majority of examples of nationalism, are the former. The "We're better than you." mentality. So again, while theoretically there could be some concept of nationalism that isn't inherently based on an Us vs Them mindset, and fosters that mindset at the expense of villainizing those different from them. It doesn't appear to be a very popular, or effective form of nationalism.
I usually use it to broadly refer to the concept of actually seeing ourselves as HUMANS, and not Americans, or Mexicans, or Europeans, etc. That since the beginning all of our actions have impacted everyone else, and our self-made society/economy, has evolved to a global level already, and has been for decades. But we're still all focused on "I'mma get my slice" at the expense of everyone else. And it's easy to think that, because "Fuck them, they're just dirty foreigners, they're not True Americans, so I don't give a shit what happens to them", is apparently a very appealing mindset for a lot of people. Because they've been taught for generations to think that way about other people. I mean fuck even just within US mindset alone, we are further divided into regions of the country, states, and again often have this "We can do it ourselves, and fuck the rest of you" attitude. Just look at Texas, proudly waving their dick about, claiming full independent sustainability, and the first crisis pops up, their infrastructure breaks down, and they start trying to blame everyone BUT their own narrow minded view of the world.
They can think that all they want, the reality is we are all dependent on each other. So, having a global mindset, and embracing globalism, means (to me at least, though I assume most probably consider something close to what I'm describing), is putting aside our petty fucking differences, stop seeing anything other than just another human being, irrespective of geographical origin, and finding a way to handle our shit in a way that benefits the most, harms the least, and isn't predicated on a system of continued consumption for the sake of consumption, leading to an inevitable systemic collapse of our own making.
That clear things up for you?