For shutting down the engine, go to your key assignments (from the main screen--click on "Settings" and the under controls click "Assignements")  and where it says "Event Category" select "Engine Commands".  Scoll down a bit and you should see "Engine autoshutdown".  This is the key assignment you are looking for.  Either use the key it specifies, or click on it to change the assignment.  Now when you fly, press that key and the engines will shut off.  To restart, check the same menu, and look for "Engine Autostart".  Use that key (usually Ctrl+E) or change it.

For moving backwards, there is a pushback system installed in FS (Shift+P) but I tend not to use that, as it only pushes directly back and you can't steer it.  There is a pushback gauge that you can add to any panel with the included instructions that I am unable to find right now, but it makes it possible to steer the plane in reverse.  Perhaps one of you other kind souls out there will be of some help?

You can steer going backwards, it just takes a fuselage length or so before it starts turning

Ozzy 8)

Okay, I found the pushback gauge.  The reason I suggested it was that instead of sitting there and waiting for the aircraft to turn (1 full length), you can pay attention to some of the other stuff needed for takeoff while this gauge backs you up and turns you for a counted number of seconds.  Very handy to have around.

Go to www.avsim.com and in the search field type in this: RCBpb-20.zip.  That will bring up the gauge.  There is also a txispeed gauge that keeps your taxispeed at a constant speed without having to adjust anything.  Really good for those who aren't really adept at ground handling.  At the same URL, type RCBts-10.zip into the search engine.  There is one other part to this set: RCBta-10.zip.  They all include instructions on how to put them in panels, so it shouldn't be too hard.