Seems like the best place for staging is directly below your yard. That way, it is accessible from the front of the layout, and all the switches are in easy reach. For ease of use, I would prefer a two-ended yard. Trains leaving the yard go straight into a reverse loop. This way you don't waste time with backing movements.
How to tie it in...? Here's a crazy idea to play around with: Put the entry into staging just above and to the left of the words "Whatever Scenery." (Will need to make changes to the pink track here to make this work...) A train moving clockwise around this curve takes a turnout to the left, the track makes a quarter-turn, and the train crosses the mainline at grade right in front of the switch tower (May need to shorten the yard a few inches to get room). The track begins to descend, ducking under the mainline that runs along the top of the layout drawing, following the same path at about the same grade, and ending up on the left end of your lower-level yard. Tricky part will be access to the descending track. Good news is that derailments there should be virtually nonexistent, since there will be no switches, and grades and curves no worse than on the top of the layout.
Trains leaving staging will have to terminate at your top-level yard. At a minimum, they need to swap engine and caboose so they can run in the opposite direction to re-enter staging. If you don't want to do that, you could try to get a wye in near "Whatever Scenery," so staging can be entered from either direction. Or else work in a reverse loop on the top level someplace, so the trains from staging can turn themselves.
Just an idea. A good one? Not sure 
