This is equivalent of using a single global lock (and STM is semantically equivalent, just theoretically more scalable). It obviously works, but greatly limits scalability by serializing all operations.

Also in practice the CSP node that is providing access control is effectively implementing shared memory (in an extremely inefficient way).

The fundamental problem is not shared memory, it is concurrent access control.

There's no silver bullet.

Yes, multithreaded problems go away on a single thread.

Is there any way for an external thread to ask (via CSP) for the state, think about the state, then write back the new state (via CSP)?

If so, you're back to race conditions - with the additional constraints of a master thread and CSP.

That would be shared ownership again.

So then I would sell STM to you from the "other end".

Everyone else has multiple threads, and should replace their locks with STM for ease and safety.

You've got safe single-thread and CSP, you should try STM to gain multithreading and get/set.

CSP suffers from backpressure issues (which is not to say its bad, but it's not a panacea either)