The sound you hear at light throttle is spark knock..detonation occurs at higher rpms, and is essentially inaudible. Spark knock is pre-ignition..spark lights off the burn a little too early. Detonation is SPONTANEOUS combustion, explosion, rather than a slow burn as is normal. The 2 ..pre-ignition and detonation..are substantially different. You probably have detonation.
Any temperature hot enough to melt an electrode is dangerous and damaging.
A plug that is loose does not transfer heat to the head efficiently, and can overheat, but usually not to that extent. I suspect you are borderline on too high compression or too advanced ignition. Or, the carburetion is extremely lean.
Proceed carefully. What do the rest of the plugs look like? Where is the temperature change line on the ground electrode?
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