Whilst I appreciate you may have been feeling a bit frustrated when you posted your message, it would be appreciated if you'd be a bit more temperate next time.
The answer to your question, how to change the mouse toggle button, will be found in the Visibility tab of Dock Settings and is fully explained in the Help Browser.
However, the premise of your first paragraph is entirely false. When Circle Dock is made visible, it will be the "active window" and all mouse input will be directed at it. To get the mouse to work with a different application on-screen at the same time, simply move your mouse to that application and click the left button to make that application the "active window" - and yes, your scroll-wheel will work as expected. Basic Windows stuff.