I think that healing could be included in Inquisition and the answer as to how to do it lies in the lore.
In the novels casting healing spells was exhausting for the mages so why not add a debuff everytime a mage uses healing spells?
I imagine that it could be something like this:
Every healing spell creates a stack of the debuff, call it "exhaustion". Single target heal creates one stack and group heal three. Make the debuff work in stages like: one to three stacks - reduced movement speed, from four to six even more reduced movemetn speed and lower damage output. From seven to ten - greatly reduced movement speed, significantly reduced damage output and reduced mana regeneration. The debuff could have a timer, like ten minutes. Every use of a healing spell would refresh the debuff untill seven stacks. At seven stacks and up the mage would need to rest for the debuff to go away. At ten stacks the mage wouldn't be able to cast any more healing spells because of exhaustion.
Remembering the lore from previous games the group heal should be a spirit healer only ability, with guardan spirit and revive. Passive could work around being able to cast more healing spells.