Your premise isn't really comparable, not least due to its unrealism. However, generically, if Ukraine invades somebody's country under similar circumstances to Russia's, then yes, absolutely.
Let me know when the Ukranian army crosses the frontier of the breakaway region. Until then, "the Ukranians invading their country" is rhetoric that is unconnected to reality.
Okay, they've just been shelling it for the last 8 years. That's totally fine until someone puts a foot over.
I think you're letting emotions migrate the topic. The implication is not that it is "fine". War is never "fine", no matter how "just". The question is that of when it becomes impossible or undesirable to police "calls for violence", which is very broad. Ownership of that area is much more controversial than the ownership of Ukraine, and the actions taken are much less dramatic. I.e. attempting to prevent separatism is much less black and white than responding to a full blown foreign invasion. However, I understand there are people who disagree with what I think is "obvious".