what would happen if a pre-veil entity and a post-veil entity interacted?

[anagogy]

for some reason this struck a chord of... fear? in me. the idea that one could view existence as "not mattering" ultimately is terrifying. But maybe that when we reach that very same conclusion after eons of believing everything is important, it's a relief and a release...

It isn't so much that existence doesn't matter to them, it's that they are perfectly content to not grow. If you give the rat the cheese before it runs the maze, it doesn't see the point in running the maze 99.9% of the time.

how, actually, does the pre-veiled entity build polarity at all?? what is their catalyst, if not from the potential for service-to-self?

They build it the same way a veiled entity does, by service to others (I would say service to self as well, but an unveiled being would have extreme difficulty becoming STS). But it is harder for them to build up enough of the requisite love because there is no essential existential angst or dissatisfaction driving them towards 'something better'. They are divinely happy, and thus are content to progress at a snails pace. This is why the veil was implemented.