The Cerbo, operating with DVCC, will certainly provide relay control for the alternator cut-off.
I believe (but have no practical experience) that the Victron BMS will interface to the Cerbo to provide the necessary signals. That is a valid solution.
However, I think that a better solution is to control the alternator cut-off directly from the BMS. This eliminates one level of abstraction and a potential point of hardware/comms failure.
The Cerbo is still a very useful device in a Victron system for all sorts of reasons that you already know!
I wonder if you actually need the BMV-712 in your system? Does the Victron Battery/BMS subsystem provide all of the same information via the Cerbo/Touch display? If so, what is the purpose of the 712? Maybe it is not required, in which case you may consider leaving the Xantrex display in-place and relocate its Shunt to monitor the 12V starting battery.