I live in Appalachian Ohio in mine country, and in some parts of our county you need a rider for mines on your home owner's policy. I have to have one at a property I own in town since that section was built over old mines. I also have an old coal bank on my rural property. It was mined privately by an individual, by pick and I'd expect it not to have conventional looking excavating, either. There may be no rhyme or reason why this wasn't just filled in and a piece of wood thrown over it and left to rot. It could be anything, actually including an old dug well or spring or rain water cistern where the bricks had been removed and used for landscaping or something, lol. That's almost the exact measurements for my spring cache basin near the house.

I really like the sofa/chaise idea in the above diagram. And the chairs could be swivels. I think you would miss the storage if you removed all the built ins. But, I would be inclined to remove the one section of built in that acts as a TV stand or at least have it reworked to match the depth of the rest of the unit. Since flat screen TVs don't need much depth, I would place the TV in the built in across the short section that would directly face the sofa/chaise, perhaps wall mounting it directly to the back of the unit and having all the shelves and vertical divider removed. I would create an additional seating area with an occasional chair in the corner and a small table under the window that is to the right of the fireplace and eliminate the secretary desk that is against the wall adjacent to that corner. Totally agree about no TV above the fireplace. I had a contractor tell me that one of his most common jobs is removing such TVs to a more comfortable location.

Perilla? What does the foliage smell like when crushed? And can you tell us the geographical location, please.

Will be on the lookout for this rare bird around my house!!!😂. If I spot one will name it Chiquita, and noticed they fly in from Guatemala.