Q3.7.7: How do I select a rectangular region?

Just select the region normally, then use the rectangle commands (e.g. kill-rectangle on it. The region does not highlight as a rectangle, but the commands work just fine.

To actually sweep out rectangular regions with the mouse you can use mouse-track-do-rectangle which is assigned to M-button1. Then use rectangle commands.

You can also do the following to change default behavior to sweep out rectangular regions:

(setq mouse-track-rectangle-p t)

You can also change this with Customize. Select from the Options menu Advanced (Customize)->Emacs->Editing->Mouse->Track Rectangle... or type M-x customize RET mouse RET.

 mouse-track-do-rectangle: (event)
   -- an interactive compiled Lisp function.
 Like `mouse-track' but selects rectangles instead of regions.