I bet the MWBC problem will be handled by a compliant AFCI that can reconcile the current properly. They already have GFCIs that do it ... at 5MA.
The actual "arc" part should not even care if it is a MWBC.
I do understand "smart" overcurrent protection, we did it on the big power supplies in our mainframes. They could detect fairly small faults on a 1.5 or 3 v supply that might be running close to 100a.
