I've not been able to get the rig to key/unkey using CAT commands. The COM port, baud rate, and other settings are correct, and are those used by FLDIGI, JTDX, WSJTX and work fine with those programs.
OmniRig works as it should too.
I have USB KEYING (CW) turned on, as required by WINLINK.
If you have been able to use CAT commands in VarAC to key/unkey the 7300, please look at your 7300 settings and tell me what your USB KEYING (CW) setting is, and also any other settings you feel might need to be changed.