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I'm not sure what the percentage healing bonus is, but almost any stone bonus is going to be better than the +magicka/health/stamina stones. You can get those resources from gear and food and easily get near the softcap. Regen and healing power are far more important for healers.
I think it's ~5%. This is what I have seen from using swallow soul with NB.I think it's maybe healing received since the skill should heal for a static 25% of damage done.
I made a quick test with the Ritual mundus stone. It seems indeed to increase the healing DONE.
However, I'm not sure how it is affected by the divine trait. Having that trait on 5 items (34% overall) just increased my heals by ~5.6% (mundus stone bonus already included)