Well, answering question 1... changing your dpi from 300 to 500 on a 3000x3000 image would give you a 6 inch square image, not a fifteen. The way to calulate this is to divide your pixel dimension by the dpi to get your print dimension. Thus, is you want a 3000x3000 pixel image to print at 15 inches square, then you need a dpi of 200.

As to the others... not done enough tinkering with the setting myself to come up with a definitive set of 'killer' settings, so can't help you there... :)

jonthecelt