Hello. I would try to recover the original CAS with these steps:
• Find good flash file from same part number CAS. Write it to CAS (you said support provided good flash so write that to original CAS).
• Read original eeprom, change line 0F90 (first three columns) to DA AA 55, write back.
• Perform individual programming (using autohex on bench)
These step should bring the CAS back to life. Now you can learn new key (sometimes you don't need DME ISN) but it may ask you for ISN which autohex can extract DME ISN for you.
BMW File Service
VAG Immo Data Transfer