recover MCUser password

Yes, it should be possible with the change-passwords script. Please check the Avamar Administration Guide on how to do it safely. Don't change the OS passwords, don't generate new SSH keys. Just the Avamar Server passwords. You will need the "Avamar Server" root password, which might not be the same as the OS root password.

https://dl.dell.com/content/docu93969_Avamar_19.1_Administration_Guide.pdf?language=en_US

It will then restart the MCS as part of the process. You would have to start the scheduler manually, but the script reminds of you of that at the end.