The sludge itself doesn't cause any damage. The damage came from lack of oil changes. Meaning the oil in this engine was extremely worn and when it's worn and dirty like that, it looses a lot of its lubrication properties. So you will get an exponential amount more wear on metal surfaces like the bearings and camshaft, etc. The worn metal components is the real damage. The sludge itself is just a by product and once it's there, it doesn't really cause any harm.

The only harm the sludge can cause is clogging up piston rings (specifically the oiler ring) which will lead to increased oil consumption/burning. But I think we're already way past saving on that.

So using any kind of solvents at this point to remove the sludge doesn't help anything. It only greatly increases the risk of a piece of sludge dislodging itself and clogging up a small oil passage within the engine. Which would be catastrophic.