Under Polish law, my understanding is that you cannot be a dual national.
Yes . You're right!
What it actually means: when you are Polish citizen and additionaly hold any other country citizenship, you cannot and should not enter Poland as different citizen, other than Polish.
While crossing Poland's border and if they (the border security) find out that you are Polish and showing other country passport, you can be stopped and redirected back to where you came from.
It already happened to many people. Just have a look at Polish embassy/consular in your country and have a look & read all the details.
Some Polish are risking (some people know, some don't) their holiday and go to Poland holding passport of the country where they currently live.
That's how it works.
On the other hand, you cannot automatically loose your Polish citizenship unless you apply in person to a Polish President and he accepts your application with payment (it used to be 1000 zlotych few years ago).To add
: the above stated regulation is directed to all the people holding Polish citizenship - not Americans only!
What about in my case, where my nationality isn't the same as my citizenship? (British citizenship, Scottish nationality)
In short, because you are the United Great Britain (or: Kingdom of Great Britain) citizen, that's why you're British and nationality depends from what "Great" area/ country you come from - in your case Scottland, in others it may be England etc.
I don't have any allegiance to the UK whatsoever, nor do I have any to the British state - yet because there's nothing such as a seperate Scottish citizenship, I have to carry a British passport until such time that it changes.
I know what you mean, but on the other hand (because of the Kingdom) you have such a great opportunity to see the world, the other countries where the Queen is a head, without the hardship others have to get a visa & enter any Queen's other countries (like NZ, AU, CA...)