I find your argumentation fairly sound and straightforward. I guess one small technicality I would pick on would be a subtle implication you make; your formula seems to imply that if candidate Y supports the greatest evil of our time, and candidate Y is elected, the evil in question will inexorably take place.

Assuming we're talking about a democracy, such implication is unwarranted. Presidents and leaders support all kinds of bills and policies that never get passed due to opposition in legislative bodies, regulatory restraints, and other countervailing political influences. In fact, if anything, the contrary could be plausibly argued: most policies supported by politicians never get implemented (hence their tarnished reputation of being liars).

Even if politician Y supports the greatest evil of our time, its still conceivable that given certain contingencies, he/she will result unable to bring it about. Furthermore, if despite supporting the greatest evil (which he wouldn't be able to make happen despite his intentions), politician Y is additionally supportive of good policies and governmental courses of action which have a higher chance to be successfully implemented, voting for him/her doesn't seem too irrational.

Exactly, "will" be affected, not is being affected.

I see where you're coming from, and surmised you would respond something along those lines. I guess the problem I would have with that relates mainly to how you construe it.

I guess Im not necessarily ready to forego the "prior existence" condition.

On the following basis:

It seems to me that there needs to be a certain sort of temporal symmetry when you affect someone.
The two agents need to exist simultaneously, in the same temporal reality before we can talk about any party affecting the other one.

Were you to claim otherwise, would that not be falling into a category mistake?

For instance, in your thought experiment, I would have a somewhat different interpretation. Whereas it seems that you would conceive that Trudy is affecting her child, I would construe it as Trudy doing something that will affect her child in the future.

It's a somewhat subtle distinction, but one nonetheless.

And no good arguing that we cannot affect people by bringing them into existence, for that falsely assumes that to be affected by something you need to exist prior to being affected. — Bartricks

This confused me a bit. How is it not the case that to be affected by something you must exist priorly?

Can something/someone nonexistent be affected by something/someone else?

What would that even mean? What would be the thing be affected if it's nonexistent?