Can't close tabs on browsers using middle-click unless the browser has focus
Unfortunately, I'm afraid you're out of luck here. OSX never gives focus to a window unless you left-click on it or Cmd-Tab to it. There were some older solutions that could provide focus-follows-mouse, though they're not supported in the newest versions of OSX.

Sadly, middle-clicking non-focused Chrome tabs used to close them. But I believe this feature was lost once Mavericks came out. You can middle-click on some other apps (like Terminal), and they'll respond appropriately. But no browser seems to do so.

As for your question about using middle-click-and-drag to scroll, that's something I dearly miss from Windows, but I haven't found a free way to get it on Chrome in OSX (it works in Firefox, though). Smart Scroll is an app that offers "Vector Scroll", which works like the feature we want. But it costs $15.