But if agent X will (deterministically) choose action a1, then when he asks what would happen “if” he takes alternative action a¬2, he’s asking what would happen if something impossible happens.

I don’t immediately see what the problem is with considering ultimately impossible choices. Perhaps considering a concrete example would be useful in triangulating towards a proper understanding.

Let's suppose I have a function F(x) and suppose the domain of x of interest to me is {-1, 0, 10}. I would like to choose x to maximize F.

My choices are x1=-1, x2=0 and x3=10. I "consider" each choice by calculating F(x) and I choose the one that maximizes F.

I would say that x1,x2 and x3 are all choices because I compared them all in order to make my (deterministic) final choice. There is nothing problematic here, so I must delve deeper...

Perhaps the problem is with the calculation of F(x1), F(x2) and F(x3). In real life, you can't calculate F(x) because doing so would require already choosing x, so you can only simulate what F(x) would give you. So here it seems necessary to make a distinction between F(x) and Fs(x), where Fs(x) is a model of F(x) that you can use to simulate what F(x) would give.

Thinking about what "would" happen "if you chose" x1 means calculating Fs(x1) -- not F(x1), because calculating F(x1) means choosing F(x1).

So maybe the naive realist is confusing Fs(x) and F(x)?