For what you want to accomplish, I agree that a short divider works best. That really only loses you under 2" of depth, when you consider the bracing needed. And, it should probably lift away to deal with track maintenance and derailments. If you really want to go to the trouble, lift-off, slightly raised scenery could hide the return path, but the liftoff would have to be broken into sections to make it feasible to really move it.
Putting it under the layout makes it a real chore to fix a derailment. And, the older we get, the lower and farther back those hidden tracks seem to get.