I've tackle the 1. by replacing write/open with os.write/os.open. I still get :
$ python3 simple_incorrect.py
b'\xc3\xa9'
Traceback (most recent call last):
  File "simple_incorrect.py", line 32, in <module>
    os.write(tun, incorrect_frame)
OSError: [Errno 22] Invalid argument

I'm going to study more in depth the point you raised about tun.c