Sorry, you can’t get there from here?
If you simply have to do it, I’d look at a helix or something similar in the lower left . Depending on the height of your backdrop(sounds like 10-12"), once the helix is complete with scenery it could easily overrun the backdrop height.
The second concern I would have would be the width of the bench work itself. If you’re considering passenger cars they may not be able to negotiate a helix based on the bench width. It seems that at 2 feet wide you’ll be stuck with a minimum radius somewhere south of 12"(most likely 9.5"), so a lot would depend on your passenger cars. Also consider than an “on layout” mountain would normally taper as it goes up cramping the radius even more. Also consider the logistics of working inside an 18" wide “chimney” to right your cars should something go awry, - unless you build a lift off mountain (which I have dine).
If it were me, I’d consider an off layout helix at the lower left of sufficient radius to accommodate your passenger cars. The cars don’t have to look realistic on the curve, but you don’t want them straining the limits of practicality either. If you assume a 17" radius helix ( I choose this radius because for practical purposes we can call this 100" per turn) you’ll still need a 2% grade +/- for clearance , so six turns. At 2% grade, the rule of thumb tends to be that your train length is halved - assuming a straight line. This brings us to the freight situation - can your single loco pull 30 freight cars on the level? If so, you MIGHT get 15 cars up the helix without helpers.
All that being said, I had an almost identical situation. My back drop height is 14" and I put in the helix off layout. The difference being that I don’t run passenger service through the helix and my freight trains are limited to 14 cars without helpers. Of course, the helper set-up just becomes an operating feature.