On the second question - my understanding is that non-union associated employees do not have a protected right to strike, as individual employeees. You can't just refuse work and say "I strike!" At least their rights are weaker. Much of Swedish labor laws are about associations of employees (=unions) and associations of employers. As an individual, your rights are not the same. We will see what the courts say in the end but this distinction is not so well known, and it often comes as a surprise for non-unionized members when a company is downsizing.

I'm curious as to whether an "association" legally implies those who have membership in a union, or more generally covers coordinated action between union members and non-members, or even coordinated action between non-members alone.

It's the other way around. Unionized workers have traded away their right to strike without union support.

Anyone can strike, it is a basic right, but good luck not getting paid. That salary might have been handy for the legal representation after being fired too.

No PostNord doesn't know and cannot ask.

Anyone can strike. Being part of the union and contributing to it guarantee you are being paid (by the union) while on strike.