A large oval cut-off can be added, connecting a yard track with the lower reverse loop under the mountain. The turnouts are actually accessible from the open mountain-end, and hidden-yard side. FasTrack turnouts are very reliable, but still, one would have to seriously test these turnouts before installing them in a difficult to reach area. They COULD be accessed and even replaced, but it might not be fun!
It would add a lot of operating variations, so might be an idea. Trains staged in the yard, could enter the layout from underneath the mountain, as well as through the right side tunnel.
And it gets you running trains on the level, while building the grade and elevated section.
I might continue the grade from the spiral loop across the elevated line at 1-2%, to gain another 2-3" beneath the mountain for easier access beneath, at 9-10" in elevation (perhaps 8.5" of clearance). Install some maintenance lighting.
