Certificate of No Impediment - a written statement from the Embassy verifying that you are legally FREE to marry.
As far as I'm aware, the embassy issue CNIs only to people who have an address in Poland. That is what a comparison of this site gov.uk/marriage-abroad/y/poland/uk/uk_england/partner_local/opposite_sex and this one gov.uk/marriage-abroad/y/poland/other/poland/partner_local/opposite_sex suggests.
4. Certificate of REGISTRATION (for permanent residents only)
No, temporary residents also need to present their residency permit.
In order to get married in Poland, one must evidence that one is legally in Poland: that means a residency permit and a maldunek, or a visa and a maldunek, or just a maldunek (if one has been at the address one gives for more than three day, or is it five?)
5. Documentation verifying that you're legally allowed to get married in accordance with your national law
How does that differ from the CNI?
Happy with the answer Harry, you are so negative and difficult. Get a life.
It's not me who is difficult (in my opinion anybody brave enough should be allowed to marry on the spot), it's the defenders of the red tape here who are difficult. Expecting the worst and planning for it are the only ways to do it when dealing with red tape here.