With regards to the redundancy behavior with dual MMs in a VSF member — it does not matter what mode the member was in prior to joining the VSF fabric as the configuration is replaced during the joining (or VSF fabric creation) process, which disables the normal dual-MM redundancy behavior and shuts down the non-active MM (which is referred to by the switch as an "alternate" MM, rather than as a "standby").
There is no requirement for both VSF members to have two MMs installed; if a second MM is installed in one member but not the other, the fabric will continue to operate normally, though only the member with dual MMs will automatically recover from a failure of the active module.
Having dual MMs installed in a 5400R VSF pair was initially considered an unsupported configuration due to uncertainty about the behavior of the second module in the event of a MM failure in one of the VSF members. This scenario was explicitly tested starting with the 16.05 release to verify that the 'alternate' MM would take over as intended in the event of a failure of the active module; as a result of this testing, we now 'officially' support having two MMs installed in one or both VSF members.