> getting involved in another unrelated unions negotiation seems to cross that line.

Companies do favours for "unrelated" companies all the time - Tesla of all companies is in no place to complain about the same tactics being used on them. Outlawing this is what turned anglo countries into the capitalist hellscape they've become.

> if one union is responsible for unloading at all ports in Sweden (idk if this is the case and it wasn't that easy to look up), then that feels very much like a monopoly.

The union represents the workers. It's not a monopoly any more than a country having a single government is a monopoly.