After a bit of help from folks at Massalit (English-language bookstore in Krakow),I was directed to where to look for Lem books in Polish, and after looking through 6-7 stores found copies of Summa and Solaris in decent condition as souvenirs of Krakow (since neither is (properly) available in English). There were lots of books about Lem (which I wonder if we'll ever see in English) but not many shops had a good selection of his actual works.
None of the bookstores had heard of Daniel Mróz, although several had a book "o poznaniu w tworczosci stanislawa lema" with a Mróz cover which I used to demonstrate who he was, and none were willing to consult their computers for any information on books of his works.
The email link to Mróz relative (daughter?) on the gallery pages here seems to be out of date too.
Will try in Warsaw tomorrow.