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.

As sg said, the pot is way too big. It needs a very small pot with drainage holes. It will not do well in that pot. Plastic is fine but I prefer terra-cotta. The pot could be just slightly larger than the width of the top of the plant, as a guide. You can repot in a fast draining mix. Most frequently recommended here is commercial cactus and succulent mix combined 50/50 with perlite or pumice available from nurseries. If perlite, rinse tiny bits out through a strainer first. Get pumice if Available. just leave the long sprouts for now. They could develop into new plants for you as they grow larger. Oh be sure the plant gets plenty of light but probably not all day sun.

You've got a Euphorbia most likely Euphorbia trigona. Looks like it's beginning to rot or has fungus. The fact that you say it's spreading is not good. It will just spread all the way up. You probably will have to perform surgery. When you do cut it. Just use a clean sharp knife. It will ooze a white substance. Reminds me of watered down Elmer's glue, but this ooze is toxic. So wear gloves and protect your eyes. Everybody responds differently, but I don't think you want to test it;). You will cut the roots off, but that's ok. Cut until you see clean flesh. The bad parts remind me of bad melon. It will be brown. There should be no brown spots on your cutting. Once cut and all the bad parts are gone you can put the cutting under a faucet with slightly warm water. This will stop the ooze. Just get a paper towel or something to lay the cutting down on, so it can form a callus over the cut. This could take a couple weeks. Once a callus has formed you may replant not quite an inch deep. You will need to stake it up till roots form. Keep in a shady area away from direct sun. No water till roots form either. Euphorbias can take months to form roots. So just put somewhere where it's protected from the cold, water, and sun, and forget about it. I had to reroot a Euphorbia this year. That plant took about 5 months to reroot. So don't go checking every week because the roots won't be there. You also disturb the rooting process by checking on it all the time. Just be patient. Your mix should be fast draining. If you haven't, add 60% perlite into your cactus mix. This will help with drainage and aerates the soil. I know this sounds like a lot, but it's the only option when you have rot unfortunately. Well there is another option. Just buy another;). Goodluck:).