>>> Can one enable Horn Master without enabling all CVs? <<<
Enabling the Horn Master CV's in QCVM does enable the CV's in the decoder and does not mean that the decoder's firmware supports these CV's.
The Horn Master CV's are automatically enabled in the decoder's firmware if and only if the firmware supports the Horn Master CV's.
>> I am familiar with feature 145 (even if I didn't use the correct nomenclature). My point, now rectified was that the ver9-2-5 firmware in this particular model did not
allow Disconnect or Standby. Double pressing F9 went straight to Shutdown. <<<
Your firmware had CV53.11.1 set to 143 "Shutdown" instead of 145 "Disconnect/Standby/Shutdown".