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Oil doesn't boil until well beyond 500?F.... some blends only hit boiling point for mere moments before flashing (exploding into flame). If the oil in your plug galley is reaching those temps regularly then save yourself the time and set the car on fire now before it does it for you and without your consent.

Oil will degrade plastics, rubbers and silicone if left in contact with those materials. It's not uncommon for them to reduce to a glob of goop resembling melted plastic. It is not going to soften the spring clips at the ends of the wires... If the wires are coming off they A) are some amazingly cheap crap; B) Are stretched out from improper installation/removal/handling; C) are too short (either in total length or the available length in the boot in which case more wire just needs to be pushed into the boot).

The #3 valve cover gasket is not designed to old in pressure. Actually the way it's designed it's almost like a check valve (only when new). Even if there were significant gas building up in there the most you'd see is the top seal of the spark plug boot popping out to let the gas vent. A cracked head, spark plug hole with the threads completely stripped or a spark plug which has somehow developed a hole all the way through are the only ways I can see a plug wire literally getting blown off on this engine.

ebang, it sounds like the vehicle may have very seriously overheated from a sudden loss of oil. Do a compression test already... details are in the TSRM, it's quite easy. Drain and inspect the oil. Use a clean drain pan so that you're sure that any grit or moisture is from the engine. Drain and inspect the coolant (you can't judge based on what's in the reservoir).