The attached patch (wrap_threadstate.diff) is enough to stop the crash. It's a slightly dirty proof-of-concept, but equally could be the basis for a solution.
The main functional issue is that there's still a race on the Py_BLOCK_THREADS side: it's possible that the "is threadstate still valid" check can pass, but the interpreter is finalised while the daemon thread is trying to reacquire the GIL in PyEval_RestoreThread.
(The Py_UNBLOCK_THREADS side is non-racy as the GIL is held while the ts and wrapper updates are done).