I don't have a way to test the spark with the engine running. But pulling the wire off the tower while running forces the spark to crossfire to the mounting bolt, which is not good for the longevity of the coil. Poking holes in the insulation is going to mess with the high voltage breakover capability of the wires. Using a test light on a circuit that has 30,000 volt spikes in it is a pretty rough way to test this anyway.
The stock ignition is a reasonable performer when in top condition. Just maintaining the parts is the first step. Corroded/rusty coil terminals is a symptom that the wires are in need of replacement, and perhaps the coil as well.
What the OP should do is remove the wires one at a time and clean and inspect them. But for a rough idle he also needs to look into vacuum leaks, BLM issues, injector issues, etc.
The RTV on that one wire means that there are probably other issues going on with them and replacing them is a good idea.
Bob