Reagarding the transponder and why it was not returning a secondary paint on radar could be as mentioned above. If a decompressurization did occur, and in the process of changing the code from assigned code to 7700, the PNF left it in Standby mode and not return it to TA/RA mode due to increase in workload or partial or deteriorating awareness due to hypoxia, it may explain why the transponder was 'off'. Quite plausable and we all know how overloaded we can become during simulator proficiency checks for similar scenarios.