What does "so to speak" mean?

It's not redundant at all, well, not if it's used correctly.

It is opposite of the phrase "so-called ~" "My so-called mum..." which means "my mum that doesn't act like a mum should". "My mum, so to speak..." is inferring that someone who is not your mum, is acting/functioning as if she were. "We do everything together. He's the older brother, so to speak, that I never had"

It's often used as a device in humor. It's a marker that signals what was just said is not necessarily what it sounds like and that you should look for the double-entendres, metaphors, euphemisms, hidden/secondary meaning or other figures of speech. It can be like a subtle form of "if you know what I mean (wink, wink, nudge, nudge)"

With regard to your examples, it is awkward when used with adjectives as in #2 & #3, but the others could work very well with the correct lead-ins.

ex1 (a woman speaking...) I take out the trash, do the plumbing, do the barbecue, mow the lawn - as the "man", so to speak, I feel very fulfilled.

ex2 So your roommate cleans the house, does the laundry and cuts your hair? Well, your "mum", so to speak, also cooks very well.

ex3 Why buy the cow when you can get the milk for free, so to speak.

ex4 My husband may not have a green thumb, but he certainly knows how to tend my garden, so to speak. (I heard a comedian say this once)

As was typing this, I noticed that almost anything that was followed by "so to speak" could be put in quotes and have the same effect without using "so to speak". Although this wouldn't exactly work if spoken, unless you make "air quotes" with your fingers. Hmm, I don't know what to call that effect...

Hope this helped, rather than confused you further...

Avatars refer to a concept that is not any different at all from your "real" name as also only concept. This is a spiritual question because, in fact, there is nothing such as a You that exists. The You/Me/I which folks think they are is a linguistic convention for something that exists only in a dualistic consciousness. Importantly and ultimately, there is not a You and Your Life. There is not God except as your dualistic consciousness thinks one up. Knowing this or even just acting as if this were true changes behavior in a positive way. Thinking there is a God usually (not always) results in the worst kind of behavior because of the divisive delusion and ultimate arrogance it is based on. The practice of remembering and reminding one's "self" this is usually called religion in the broadest sense. Kshetra means a field of action and a holy place - sometimes site of a war - which affects consciousness in the way that physics describes reality. The World Trade Center Site in New York is one such Kshetra.

What a great question, and it is a redundant phrase that will annoy educated persons.
Using your examples above only the first and fourth are how it is used.
1)I am a man, so to speak (denotes sarcasm) this means I am a man, but maybe you look like a woman.
2)Your mum, so to speak, cooks very well (sarcastic in saying, your "so called mum")
So I would say "so to speak" means that you or i am lying to you, but let's just ignore that little fact.

mine:http://answers.yahoo.com/question/index;_ylt=AhzZTw6xirFk52j4wZW8FZDsy6IX;_ylv=3?qid=20130512175906AAsjBGT