It does not count as Endurance Threshold for any effects that scale off of Endurance Threshold other than the damage calculation itself. We cannot make it actual Endurance Threshold because that’s a precalculated stat, so it’s only updated on stats update (e.g. when you equip an item or receive a debuff), and as such Endurance Threshold itself cannot scale with missing mana.
No, 'cause that’s not what it’s doing. It’s giving the % DR from Endurance to more HP than you would otherwise get it applied to based on the missing mana. It’s not adding additional hp.