> 1. It crashes with "KeyError". ...

I assume this means it raises a KeyError when given a bytes object as an argument.

>>> Internaldate2tuple(b'INTERNALDATE "01-Jan-2000 12:00:00 +0000"')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "Lib/imaplib.py", line 1326, in Internaldate2tuple
    mon = Mon2num[mo.group('mon')]
KeyError: b'Jan'

> 2.  The sign of the TZ offset ..

Once Mon2num is fixed, the sign error show up as follows:

>>> Internaldate2tuple(b'INTERNALDATE "01-Jan-2000 12:00:00 +0500"')[3:6]
(2, 0, 0)
>>> Internaldate2tuple(b'INTERNALDATE "01-Jan-2000 12:00:00 -0500"')[3:6]
(2, 0, 0)

 
This looks like a 2 to 3 port oversight and we can probably fix it in RC.  Georg?