2. According to Communist theory, as the proletarians (workers) gain supremacy, how will this affect the status of countries and nationalities?

I suppose it would make them by making them stronger. Consider Communist China. The whole idea is the power of the working people together in solidarity making the country powerful. I suppose this works to an extent. China is not only an economic power but a military power as well; nobody messes with China. Still there are plenty of communist failures largely because equality really isn't equality of the working person. China also has large inequalities but people don't question the system.