“2) Or will the pattidana have no impact on Y and Z’s good kamma beeja. Rather, the only impact the pattidana will have is on the mindset of Y and Z, by elevating their mindsets?”
That is the main point. One cannot “give” causes(kamma beeja). One can give only conditions for those kamma beeja to germinate. That is what I explained above and also on those posts.
However, I forgot to mention something important. Pattidana is more effective for only certain beings: Especially some pretas, and also gandhabbas to some extent. Those pretas are totally dependent on pattidana. I have talked about this.
Unless one is giving merits to a deceased person or persons, it is always better to give merits to ALL BEINGS. That will help make the conditions better in the whole world for all beings.
This is what the Arahants do, and of course they are very effective. Cultivating metta to all, and giving merits to all, will make conditions in the “whole world” better for all beings. After all, any living being has been one’s father, mother etc in this beginning-less rebirth process: “How the Buddha Described the Chance of Rebirth in the Human Realm“.