[Taqwus]

Commanders healing others in the province isn't automatic -- you need to use a specific command ('Heal Troops'; heals commanders too; probability seems unaffected by number of troops in province), so there's opportunity cost.

Arco Priestesses, a T'ien C'hi immortal national hero, some priestly type belong to EA Ermor (Flamen? Haven't tried them) can do it. There's a Sauromatian national hero who's healer, too. Faery Queens might be the only healers available to all nations without use of charming/enslaving magic, and are not limited in number. None of them can heal the undead/lifeless, if memory serves. There may also be a restriction on healing afflictions from old age, although I've not really tested that.

Gift of Health is the only spell that'll do it, and it works on friendlies throughout your dominion. The Chalice heals units and commanders in the same province as the commander who bears it, regardless of dominion. Both heal any type of unit or commander.