I don't understand what, if anything, you would consider non-arbitrary.

I'm not sure this is actually an important disagreement; I'm ok with dropping it if you want. However, you are the one who suggested that entropy could be calculated in a non-arbitrary way; but I don't think you've offered an example of such a calculation.

And why does that conflict with what anyone says about the "arrow of time"?

It conflicts with the notion that entropy is a good way to consider the problem; entropy is a non-full-information heuristic that doesn't appear in the actual laws of physics.

neither of us is a quantum field theorist

Well, I'm not a theorist, no. I do have a PhD in experimental particle physics. I will admit that the QFT classes tended to fry my brain like an egg, which is one reason I went experimental.

so far as I know no one knows how to do the QFT calculations on anything like the scale required to understand what's happening when you fry an egg

That's true. I do think, however, that an intuitive understanding is sufficient to get a grasp of how a microlevel asymmetry can become macrolevel.

Do you have any actual evidence that it's so?

It seems that such evidence would have to be in the form of simulations or calculations, since you can't very well turn off the weak interaction and see what happens when you fry an egg without it. I am not aware of any such calculation, no. But, again, there's such a thing as a qualitative insight.