Both screenshots show Windows 7.
Anyway, there's an issue with the "show desktop" button on the classic theme.
It should work fine with the modern theme.

I don't know why it doesn't work properly for you. I've just checked, and it works fine for me on Windows 7.

See my reply here:
http://rammichael.com/7-taskbar-tweaker/comment-pa...

This is classic theme:
http://3.bp.blogspot.com/-zqGiJCBubZI/Td51yumWL_I/...
This is modern/aero theme:
http://4.bp.blogspot.com/--VAFXW8L0IA/Td51yN6qm0I/...

Windows 7, classic theme?

What is "the first pixel of the button"?
How is your question related to the tweaker?

The "multipage_wheel_scroll" option, as well as other scroll options, works globally regardless of the focused window. Ctrl+Alt+Tab uses a regular window, and mouse scroll events get stolen from it.
Nevertheless, I agree that when the taskbar switcher is open, you are probably messing with it and not with the taskbar. There shouldn't be a problem to whitelist it.

Same as this?
http://tweaker.userecho.com/topic/352629

Do you have the "multipage_wheel_scroll" advanced option enabled?
Or any other mouse scroll options?

I can't reproduce it. Also, the recent update didn't introduce changes regarding the volume control.
Perhaps you're using the "Cycle between taskbar buttons" option or the "multipage_wheel_scroll" advanced option.