Neither the Justicar nor the KotF casts the psychic power. It is the unit that casts the psychic power via BoP. The unit uses the Justicar's/KotF's Ld if he is still around, an exception to psychic rules. The Justicar/KotF takes the first hit if PotW strikes, but the unit with BoP still has access to its psychic powers, even if the Justicar/KotF goes away.
For Justicar Thawn's retinue (GTK), he take's the hit of PotW, RAW. The good news is that if he resurrects afterwards, Thawn is still a L2 Psyker on his own. And the unit still has BoP with access to one power per turn.
For Mordrak's retinue (GK), there is no Justicar/KotF. RAW, a random non-Mordrak model takes the PotW hit from a psychic power cast through BoP.
For psychic powers, Ghost Knights (GK) and Grey Knight Termies (GKT) have access to all of two, Hammerhand and activating NFW. These are powers Thawn and Mordrak already have. And the exact wording of these powers "all models in the unit/all of its force weapons" indicate that both Thawn and Mordrak benefit even if either power is cast via BoP. The independent character restrictions of NFW is circumvented for both Thawn and Mordrak.
So the benefit is that Thawn and Mordrak gain access to Hammerhand or NFW activation without burning their own PML activation via their retinue's BoP rule. This becomes more apparent when the Brotherhood Banner comes into effect, giving an auto-cast power on the NFW.
I also believe, RAW, that the reverse is also true. If Mordrak or Thawn activate their NFW or Hammerhand via their PML, the entire retinue benefits. And the Brotherhood Banner can let Mordrak and Thawn auto-activate NFW. But Mordrak and Thawn will actually have to wound with their own hits first, neither Mordrak nor Thawn can activate another retinue's model's wounding hit because neither Thawn nor Mordrak have the BoP rule.
What I don't belief is that GK through BoP casting Mordrak's Psychic Communion. As a single psyker, the GK in Mordrak's retinue do not have the Psychic Communion power, so, as a single psyker, it (the GKs of Mordrak's retinue counting as a single psyker) cannot cast that power.
Otherwise we would see abuses like L3 PML Librarians in units casting 4 powers a turn and letting the Justicars/KotF take the PotW hit.
What I do believe, through the RAW, both Thawn's and Mordrak's retinue have a legal double opportunity to either activate Hammerhand or activate NFW, once through BoP and once through PML. This can benefit these units in the face of anti-psychic tactics.
Ugh, the legalese is making my head spin. I wish GW would FAQ this, but RAW, this is what I see.