Larry, working within your constraints I'd use an O36 curve in the tunnel because you can't see it anyway. My curves and switches are all O72 except for the curve under the mountain which is O36.
Regarding the closet, if you wouldn't mind the appearance the bifold doors could be removed without affecting functionality. Having done that a single track would fit on a 1X6 shelf against and across the closet opening, again without affecting functionality of the closet. As well as increasing the total length of track on the layout this would give you more space in the middle for office-type activities.
The PER is my first model railroad and as such I've been learning-by-doing. One element with serendipity is the curved tunnel-under-a-mountain and I highly recommend it. The train entering the tunnel in one direction and exiting in another turns out to be aesthetically pleasing and it never grow old.
I have learned that for me the backstory is an important element. That is, the model comes alive because it has purpose, the movement of goods&commodities to-and-from customers and the outside World. A cogent backstory, very basic-(even schematic)-scenery&structures and close-to-scale locomotives&equipment allow my mind to suspend disbelief and the trains and what they are doing become in an important sense real.
This is just me working with my gained knowledge: In your space I would definitely have the tunnel in the Northwest corner, a hinge-up bridge in the Southwest corner and a single track on a 1X6 across the South side in front of the open closet. I would have just two [long] stub sidings, one across the North wall and the other along the East wall. There would be customer car-spots the length of each siding and most of the operating activity would be (as it is on the PER) sorting&drilling of cars to serve the various customers. Whereas on the PER the Interchange fiddle-track (where outgoing trains are broken-down and incoming trains are assembled) is in an adjoining room, in your space I'd just do the hand-fiddling on the track in front of the closet, perhaps with one closet shelf reserved for cars from-and-to the outside World.
Oh, and I would definitely have my favorite accessory working hard during many operating sessions, the Lionel 282 Portal Gantry Crane:
I have added transverse movement to this venerable specimen. Actually loading "scrap" steel and having it whisked away into the outside World certainly adds to the fun and suspension-of-disbelief.