I found the answer to the SIM validity question. According to
O2 | Is the validity of a prepaid SIM card limited in any way? - FAQ "
A recharged SIM card is valid for at least one year depending on the sum it has been recharged with. It is necessary to recharge your credit by CZK 500 at least once a year or CZK 200 every half a year so that you can keep on using our services". However it's not clear what happens with a new SIM card to which no credit has yet been added.