A: Does this include Force of Nature? If so, give the math please.
Yes. It increases the amount of healing Spirit Visage gives you by 20%. If FoN usually gives you an additional 10 hp/5 it will now give you 12hp/5.
B: Does this stack additively with perserverence? AKA 20% + 4% = 24%?
Sort of. At least the regeneration part. Though from what I hear the mastery only applies to base regeneration, while Spirit Visage includes items and runes.
C: Does this count for lifesteal and spellvamp?
No. Though Riot is considering adding this in the future.