Since a garrison needs at least one unit in it to work, I tend to build either the militia or slinger buildings in my games just so I have a cheap unit to keep in my forts, but I just had a thought. If a fort has Healer in it (I'm referring to the specific unit here, but the question stands for anything with the Healing ability), other units in the garrison benefit from the improved passive healing rate, right? And since units in garrisons already heal faster, I'm wondering if it would be worth sticking a Healer in my "supply depot" provinces that have Forts and Storehouses so they could also act as a sort of hospital for nearby wounded units. Thing is, I'm not sure if this is one of those ideas that sounds useful on paper but isn't really worth it in practice. Any thoughts?

As long as I'm asking about it, I've also been wondering if that passive healing from the Healing ability stacks with higher levels and/or several units with that ability in the same army.

I really wouldn't advise placing a healer in the garrison; normally my forts contain a SINGLE militia. BTW I normally use forts sparingly, only at choke points generally. I would rather spend 500g on better units, buildings or hero equipment (assuming you have all resources; sometimes the cost can be much higher).

Also healers only increase healing of the group they are in; normally one healer should be enough as long as you keep rotating troops in combat. Also, later on you can recruit monks which heal at distance [so can fairies and clerics, but access to them is harder].