Thanks for your useful reply.
I have sent Urząd Stanu Cywilnego w Katowicach the official request using the official form (in Polish, etc.) and paid the fee, and have included a copy of the death certificate. There was no problem in data protection (even if I sent it after 25 May). I did receive a formal reply by letter as stated above. Of course I can only guess about their motives.
I will contact the archives of the diocese, thanks for sorting that out.