Surely we can talk about rational agents in other ways that are not so confusing?

Who is sure? If you're saying that, I hope you are. What do you propose?

Either way, just because something is mathematically proven to exist doesn't mean that we should have to use it.

I don't think anybody advocated what you're arguing against there.

The nearest thing I'm willing to argue for is that one of the following possibilities hold:

• We use something that has been mathematically proven to exist, now.

• We might be speaking nonsense, depending on whether the concepts we're using can be mathematically proven to make sense in the future.