Its hard, very hard to sum my thoughts of the subconscious in a single post, but I will try to answer some of the questions you came up with:
a) The SC part of our mind is basicly the instinctual part in our mind - It never shuts down (unlike the conscious part)
b) Your SC never stops to think - in fact, most thinking takes place in the SC.
c) When driving a car, for instance, your conscious mind gets into a situation of very little work, meaning that your SC does all the thinking - from driving, to whatever else it is you think about while driving (Unless talking to someone or something, which causes your conscious part to be more thinkative)
d) Your SC will never fully agree with your choice, since it almost always wants immediate statisfaction, like a little child. In fact, the whole process of “growing up” can be seen as a process of “Growing a conscious”.
e) Your SC can also be seen as a layer between the brain (the “thinking machine”) and between you, the conscious - Like a computer OS, if you will.
f) When processing an idea, you actually shape the SC thinking in a certain form: Whenever you think about anything, you are actually just causing your SC to think about it. If you get inspired by something a friend said, your SC process the words, and on the way shows up to you any ideas that might be connected. Then, you choose the idea you see fit, and direct your thoughts, the SC thinking mechanism into this idea.
So, to answer your questions: it is only a semi seperate (IE, seperate in the finished product) from you, your conscious. You can contact it whenever you let your conscious get “shut down” (Through sleep, meditation etc…). It won’t always agree to your decisions, but it will have similar think patterns to yours (Like taking your thinking patter and disolving it to pieces, having the SC think in pieces instead of whole ideas)
Whoa! Hope I explained myself well, without ranting too much. (Although I afraid this is already too late…)