Here is a crazy cool idea:

Ditch Cleric Stance, give all healers a passive trait that increases their damage the closer the party members are to full health. If everyone is at 100%, full damage bonus. If anyone is dead, no bonus. This would make sure that a healer's first job is always to keep the party healthy, and if everyone is super healthy, then they are encouraged to deal damage. You also get to blame the DPS for stepping on the AoEs and mechanics for the healer's low damage output.