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I like the pragmatic approach you take to mocking here (especially consider we both know they are BAD!) In particular I think the messages framework is a great way of making the point.
I think a bit more explanation for the Django bits wouldn't go amiss - and as I say in the comment, perhaps even ditch the whole authentication backend for a lower-level approach that uses the session framework directly.
It might also be worth drawing the distinction between mocking and monkeypatching, and the trade offs involved with each. Maybe also some more strongly worded advice against directly monkeypatching attributes in tests without tidying up afterwards.