and therefore the thread is free to use it any way it likes without having to synchronize access and without danger of stepping on another thread's toes.

Did you look at the OPs code?

His requirements mean that he has that ability--safe access without synchronisation. And without the need to duplicate through freeze/thaw (twice). That's part of the benefit of using threads.

You've sent the thread some work to do,

No need. It is a thread, It already has access. All you need to do it telll it what to access.

Using threads like processes, is like buying a moped and pedalling it everywhere.