Just to chime in here with something that caught us unawares, the WAN failover is not instant. There is a timeout and algorithm that runs that determines based on DNS responses if a WAN port is up and functional. This can take from 5 to 10 minutes.
A physical failure i believe is instant, but its been a while since i tested that.
What you might be able to leverage is a DIA style connection from your WAN Providers, where they will present perhaps a HSRP backed gateway to your MX's.