There's a risk that if you have an equipment swap or something the day of travel, the family of 4 will be reassigned to non-MCE seats and the platinum+spouse will be in MCE (or will have to fight for MCE). Saying the reservations were "linked" to a gate agent dealing with an equipment swap is unlikely to help anything, just from personal experience. But that's also a pretty low risk.

As for the bag fees, all 6 passengers' fees would be waived if they are on the same reservation. No way to do that otherwise.