If I use a healing mage I tend to set 2 tactic as:
Ally: [Status] Dead -> Use: Lesser Lyrium Potion (Advanced Tactics)
Ally: Health <20% -> Use: Lesser Lyrium Potion
Lesser Lyrium potions are cheap to make and they provide just enough mana to use all healing spells including Revive
The cooldown in conjunction with the little mana the replenish means it will take a lot for your healer to "over use" the potions when your mana is full.
This works perfectly for me.
I've also been know to use:
Self: Mana or Stamina <X% -> Jump to tactic X
"Jump" past the offensive spells so the last of your mana is saved to heal.
Just re-read your question.
I use my healer as a debilitator too (never for offensive spells), this means they do gain an enemy target, which is when they start using their normal attack.
Using offensive spells tend to draw the enemy, so I usually add a tactic at the very bottom:
Self: Any -> Wait
Which means absolutely zero action will be taken beyond the tactics
(This one may only be available with Advanced Tactics mod - not sure.)
Or, yes you can just take control of your healer. I tend to leave them to it, micro-managing a healer is very dull.
And I believe your second question has been answered.
BUT...
Unlimited Flasks: Widely Sold/Party Camp
Unlimited Elfroot: Dalish Camp
Unlimited Lyrium Dust: Circle Tower