If the soul thinks on its own but only gets to communicate to other souls via a body, you get the same observational consequences. If souls have their own language of thought, then thinking might still be mediated by language and yet that mediation not require a body.
I haven't said "thinking is mediated by language, therefore souls need body to think". I have said "if souls need body to understand and actively use language, then souls need body to think".
One question you might want to ask here is whether you are interested in going after the best possible soul theory or going after the probably-confused beliefs of typical soul-believers.
To answer your question reliably I'd need to know what do you mean by "the best possible soul theory". If it means "the theory which is most carefully crafted to avoid contradiction with observational evidence", then I am probably more interested in beliefs of typical soul-believers. Moreover, I wouldn't call the overfitted soul theory "best" and the ordinary beliefs "confused"; this creates impression that theology clears up confusion of folk religion. My opinion is that theology only replaces simple confusion with elaborate confusion of greater magnitude.
I wonder whether anybody would start believing in soul if the idea was first presented to them in form of "the best soul theory".
What I mean is just to give your opponent the best possible position reasonably consistent with his or her assertions. In other words, be charitable. The reason is that if you knock down a weak version of your opponent's thesis, you leave the stronger versions on the table. Why not knock them all down by taking on the strongest possible version of your opponent's thesis?
...Moreover, I wouldn't call the overfitted soul theory "best" and the ordinary beliefs "confused"; this creates impression that theology clears up confusion of folk r
No plot spoilers here, just wanted to flag a bit of poor reasoning that shows up in Chapter 39:
This is a surprisingly common fallacy. Just because X depends on Y, it doesn't follow that X depends on nothing but Y. A phenomenon may involve more than just its most obvious failure point.
To illustrate: Suppose I'm trapped in a box, and my only way to communicate with the outside world is via radio communication. Someone on the other end argues that I don't really exist -- "There's no person beyond the radio receiver, for if there was then there wouldn't be any such thing as damaged radios!" Pretty silly, huh? But people say this kind of thing in defense of physicalism all the time.
(N.B. This is not to defend the existence of souls. It's just to point out that this particular argument against them is invalid.)