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Regarding your point 1 and 2, they are correctIP address of CIMC and management 0 are different. Hence in a HA pair, there would be three IP addresses in play 1. Primary CIMC IP address 2 Secondary CIMC IP address 3 Management0 IP address ...
Cisco doesnt recommend disconnecting control / packet adaptors in L3 mode. Disconnecting control adaptor even when L3 control interface is mgmt0 will fail HA.