The small black areas are old damage. Leave them alone; the plant has already sealed off those areas, and digging around is only going to make new wounds. The discolouration between the ribs looks like a sort of corking, and while it's not dangerous in itself it's a sign that the plant isn't as healthy as it could be. Some of it has been there a while, as you can see from the cracks. You don't have to cut spines off to explore the large discoloured area -- use a blunt pencil or toothpick or something to feel the area indirectly. Outside is good (it hasn't been getting enough light lately -- see how the top of the plant is reaching up and has fewer spines?), but miracle grow anything is unlikely to drain well enough unless you have the bottom of the soil in contact with the dirt outside. Careful with that new growth though, it's especially unused to outdoors sunlight and will scorch quite easily. Consider getting a scrap of shade cloth and hooking it loosely over the top growth. Eventually that tender growth will no longer be at the growing point, and scorched shoulders are less bad for a cactus than a scorched head. Overall the whole plant is a bit scabby, but once you get it into the right conditions you'll be surprised at how quickly that few inches of bad skin disappears into the base. The O. trolli below is hardly a show specimen, but I'm showing it to show what the top of the plant should be shaped like, and how much increased light levels will change the spination.

wow, my first post on this website :) the 60W bulb (i'm assuming it's incandescent) isn't, imho, nearly enough light. i'd definitely change to fluorescent lights, because you can keep them closer to the plant, too, without overheating it. i recently started a citrus from a store-bought organic grapefruit. i keep it near a window to steal as much light as possible from mother nature, and also have fluorescent lighting on it since i keep it with my tomatoes. the growth has slowed down considerably, though, since i took it out of the carnivorous plant terrarium it was in, and in there it was getting light from 2 shoplights about 8-12" away for 18 hours a day. btw, if you have a camera with a light meter that gives you the f-stop and shutter speed, there are web-sites that tell you how to convert that to foot-candles so you can test your light to see if it's bright enough. (however, i've run across one that's obviously wrong so look around.) unfortunately i didn't bookmark it.

I don't know where you live, but in most cities they have businesses that provide sanding service. They have giant belt sanders that they can run your project through and make it flat and level on one side or both sides. You need to tell them you want maximum thickness. If you do this, be careful you don't over do it and have it sanded too much. You could also mortise in a slice of a red oak branch to use as a shim. Are you sure your piece of wood is dry? I hope so.

Great. I didn't know there was a generic Japanese Maple species. I'm in the process of acquiring a couple of more JM's. Is $400 a reasonable price for an about 12 foot tall bloodgood JM in a 32-gallon wood container? That's the biggest Bloodgood at my local nursery. It looks really nice. I wonder why it hasn't been sold as it's been there for a while. The person there told me that I could plant it right away as it's been under full sun for a while. They have unlimited warranty, which would really help in my case as I'm new to this. I also really want a Coral Bark, but the only shady area around my house is along the garage where there is a very big palm tree with a very long root system. So I don't think the soil there would be good enough for a Coral Bark? My only option would be to plan it in full sun. However, I don't think Coral Bark like full sun. Could someone confirm that a Coral Bark would be out of question in my situation?