Ever try just plain old lemon juice?

My old boss in Dayton always swore by a 50/50 mix of acetone and automatic transmission fluid for frozen screws. That stuff creeps into threads like nothing else. I let it soak for a few hours, then use a properly fitting screwdriver with a wrench on the handle for extra twist. @angela_allen53 has the right idea with patience and oil, you really can't rush it. I've also had to carefully pick out old threadlocker with a dental pick before the screw would budge. Heat is such a gamble unless you know the exact plastic type, and I usually don't.

Yeah, I learned that lesson the hard way too. Sometimes the cure is worse than the disease.

My first foreman in Spokane had a rule about wire nuts. He'd make us twist them on, then give them a sharp tug with lineman's pliers. If it came off, you did it wrong and had to redo the whole splice. Seemed like overkill until I helped fix a lighting circuit where half the joints had failed. That tug check takes two seconds and I've never had a call back for a loose connection.

Right? @the_oscar, my cooking is the underrated part.