About the sensitivity part, my apologies if I'm just stating the obvious, but there are two places where you can fix that, did you check both? One's on the tablet settings itself, of course, but also, in Photoshop, when you have the Brush tool selected, there are settings for it that appear on the menu/toolbar. Under that, there's an option called shape dynamics. If you don't have that checked, you won't get any variation in line width or lightness even if you're using the tablet.