Return of Reckoning

Player 1 is a shield-less tank with P parry.
Player 2 is a magic DPS with dP parry strike-through and dD disrupt strike-through.
Player 1 guards player 3.
Player 2 hits player 3 with a magic attack.

What is the chance for player 1 to parry the guard damage?

A: It's just P.
B: Between P/2 and P - dP.
C: Between P/2 and P - dD.
D: Something else.

The only way to avoid damage from guard abilities is through blocking, parrying, and absorbing.
The source of the damage you receive (ranged abilities, melee, or type: magical or physical) through guard does not matter.
They are treated as melee.
So in your case, a 2h tank can only parry.
This means that only parry penetration is important.

I would like to add, since not everyone knows this (!), that damage from guard is not mitigated. This means that no matter how much toughness, armor, or resistances you have, the tank always receives 50% of the damage that hit his guarded companion.

Good question.

It should be either MAX(P - dP , P/2) or MAX(P - dD , P/2), but I'm not sure either which is the case.

The same question stands for dodge too:
e.g. a Shadow Warrior doing a ranged attack on player 3.