The soft or rounded edges will come from light creeping in around the edges of the art....which probably means that you need better contact between the art and the screen. Do you have a piece of plywood wrapped in a soft cloth, or something like that, which fits the full inside dimensions of your screen? Without a vacuum blanket you definitely need that at a minimum, I would say. Just making sure there is enough weight on it to keep it flat.
Shouldn't need to decrease the distance between the bulbs unless you are seeing some 'banding' on the screen (less exposed in the spaces between where the lights are).
Re: bulb distance from the glass.....I have it set somewhere between 2-2.5". This was the recommendation and it works great.
One other thing....as you progress, try to find a more transparent solution for your artwork (film). This will drastically reduce your exposure time and the problems associated with it :] Less time = Less light creepage.