Do I want to apply the halo to the fill or the outline?
You want to apply halo to your fill. Black outline, black fill with a grey (or white) halo effect. And yes, style effects are only visible when rendered.
Let me ask you guys this: is there a way to tweak the halo so that the "ring" can have a thickness value of less than 1? It gives me the error "boop" when I try to input a decimal, so I'm guessing no.
Also, another problem I came across with the halo solution was that even when I hide an edge, the halo is still there, know what I mean? I wish you could hide the edge/ring of the halo between two given points just like you can with the actual outline.