Cannot answer anything about promo cards.
Healing restores HP whether through fountains, items, or abilities. Scars reduce the maximum HP. So scars cannot be healed.
My logic behind this is as simple as you stated:
The rules (at least the English ones I am looking at) don't seem to specifically say whether scars can be healed or not.
If it's not in the rules you can't do it. BTW the Cranio Creations website has an updated English rules pdf.