I suspect most of these issues will work there way out with practice and experience.
Problem 1 - Not entirely sure what you are describing - perhaps you are not overlapping your strokes enough so that you are having some areas well covered and others not to well covered.
Problem 2 - Within limits, you can open up the needle and add more pressure for a wider pattern. After a certain point though, you would need a larger needle. Experiment on a scrap model or a sheet of styrene plastic.
Problem 3 - Usually I try not to make too many passes over the same area. When using flat enamels, it seems like things are going best when the surface goes on kingd of shiny and stays shiny for just a second or two before air drying flat. Bottom line is that it is better to shoot a couple of thin coats rather than one thick coat.
Problem 4 - A grainy surface I think comes from having the paint dry in the airstream prior to hitting the model. Try moving you AB closer to your model. I think it can also come from having your paint too thick. Try adding more thinner. For me, getting the right thinner to paint ratio is the most irritating aspect of air-brushing, but it also seems to be one of the most important. Unfortunately, since most brands of paint seem to have a large sample to sample variation in paint thickness, you really can't simply add X parts thinner to Y parts paint and have consistent results. Again, the more you do it, the easier it will be be.
Problem 5 - It sounds like you do not have good ventilation. Ideally, you should have a spraybooth with some sort of exhaust fan to clear the fumes. You can either buy one or make one.