0.5% per 1% missing. so 50% missing mean 25% damage less taken.
75% mes 37.5% less damage taken.
99% means 49.5% less damage taken.
If you already have 49.5% less damage taken, I don't think you need to reduces by an extra 50% the damage taken with Final Stand ability.
About the attack speed, this seem random. I think you should remove that.
I like the idea of final stand, and it sharing a cooldown with ardent defender. I don't know if 10sec invincibility would trigger some abuse on some bosses (someone that know the game better than me may know).
May be too powerful in pvp ?
When your health drops below 0 you heal 1 damage
Replace the wording by "Prevent you health from falling bellow 1".