Please don't take my analytical approach to mean that I don't care about Kyouko and Yuma. Rather, it's because I want to help them as much as possible that I want to analyze our reasons so that we can choose the best possible approach for the best possible reasons.The issue here is that much of the payoff is entirely beside the point. Whether it contributes materially to them being our friends or not should not weigh on our decision to help Kyouko, because our aid should not be contingent on the recipients being our friends afterwards.
People are complex multifaceted creatures and can have complex multi-layered reasons for doing things. Figuring out the exact ratio of one reason to another is an exercise in futility, so for the purposes of this discussion, when I say "motive" or "reason", what I mean is that we expect an action to have a particular outcome and we desire that outcome to occur.
Motive 1: We expect that giving a grief seed will keep Kyouko and Yuma safe, where their lives would be in danger otherwise.
Motive 2: We expect that giving a grief seed will make Kyouko and Yuma more positively inclined toward us and more likely to be our friends in the future.
Motive 3: We expect that giving a grief seed will make Kyouko rethink her worldview with respect to altruism.
The first reason would probably be enough reason to take the action even if the other two motives were absent or contradictory. But the fact is that these are all complementary motives. The ratio is not important: for this action all that matters is that these motives are all in agreement and are sufficient reason to take the action.