Hi Gavin
In HA configuration, HA traffic is done internally if "one box" HA
e0M can be used for hardware assist mechanism and management
it's a combo port you also have SP (for out of band management)
Your LAN config is an option yes
Depending your Juniper configuration and possibilities, an alternative configuration would be: one VIF including 4 LAN port and create on that VIF 2 VLAN for CIFS and iSCSI
Regards