I think the start button painting margins started being ignored on Win10 (maybe Win8?).
I think your start button is not as tall as your taskbar is painting and it's getting centered vertically. You could make the button as tall as the taskbar to cover the top and bottom. If you make it taller it will get shrunk vertically to fit.
I don't know of any way to get rid of the space between the start button and the first taskbar button, I'm pretty sure you're stuck with it.
Given what you're trying to accomplish, the easiest thing to do might be to build the black area directly into the taskbar and lock the area in using the left painting margin. It would probably take a couple of tries to get it exactly as wide as you want it.