Did you discuss this with any lawyer here in Poland
My mom did and she said that the proof of the origin of funds may be required (although not always requested), as well as that I will have to give my mom special authorization so that she can sign documents in my place (called Apostile or something like that), which has to be signed by notary and government authorized official (who I still have to find...), but apparently Polish Consulate should be able to provide the list.
This is apparently because a flat is not only an asset but a liability so you can't just buy a flat for somebody (or make them co-owners) without them formally agreeing or participating in the process. It makes sense, but it all seems like a bit of pain in the a** so I might just give my portion to my mom as "darowizna" as far as i know this is not taxable between first tier family members (but I might be wrong).