Wow, 9 days is a long time. The fact that it is not responding to pg_terminate_backend means it's not even calling CHECK_FOR_INTERRUPTS inside the WaitForParallelWorkersToFinish loop, so something is seriously wrong. If the PID is still there, please do what Laurenz suggested:
strace -tp 356274
Is the problem reproducible, and has it happened more than once?
Cheers,
Greg