[Great White]

From what I gather, you say that the spark on #5 looks/sounds like it's more frequent than any of the adjacent cylinders at any given RPM, and provides no change to RPM when unhooked?
That coupled with the carbon build up is the only thing that really narrows this down to cylinder #5...
This sounds to me like cross-triggering either between coils, or plug wires, or within the #5/8 coil. If the #5 wire or coil has compromised insulation, it may be intermittently shorting to ground as well. This would all explain why it's so sporadic/intermittent. If this issue was occurring before you replaced all these parts, it may be an issue in the primary wiring before the coils.

If in fact this was occurring only on #5 before the parts were replaced, I wonder if there's a mechanical issue on cyl. 5 like a dead lifter, burned/bent valve, or wiped lobe.

Is the plug on 5 tan, black, or covered in raw fuel?