avoid cross thinking
The trick to all the more complex area of the meteorology is - Think simple
One thing at the time, in real life all factors come together and complicates the issues so much that not even the most modern supercomputers can keep track of it for more than a very limited period.
For the first question - it is very hard to answer any question with why because the whole theory is just a model to explain what is happening (how). The theory was built on observations of real life and therefore it makes sense that the theory somehow results in something close to reality. The calculations were so to say made up based on the answer. The answer is that if the forces would not equalise, the wind wouldnt follow this pattern (which we know it does) and this theory would not be good enough to appear in the books.