Ugh it looks like an inverter microwave do yourself a favor and stay away from them way too failure prone. Get one with an old fashioned 60 hz transformer if you can find one. Just the manufacturers way to reduce the copper used in the step up transformer (because it's switched at a higher frequency) at the cost of complexity and reliability.
Arcing? Sounds like the magnetron is shot just replaced mine earlier this year it took out the rectifier too with the cover off I could see the blue light through the vent holes while it was running it was arcing somewhere inside.
No they don't always fail totally short circuit between anode to case or heater to case and can't always be tested with the resistance range of a multimeter sometimes live testing with the cover off is necessary
. The transformer also buzzes like an angry hornet when there's a short circuit on the secondary
.
Unless it's an expensive over range model like mine was it's probably not worth the cost to repair.
The RF energy comes out of the magnetron antenna and goes straight into the wave guide the magnetron has a mesh gasket around the antenna to prevent leakage between it and the wave guide so yes you can operate it with the outer housing off. The wave guide then ends inside the cooking cavity it's under that shiny (mica?) sheet that RF can penetrate some pricier models have a metal stirrer blade driven by a motor just above the sheet to more evenly cook the food.
The only real danger with it unplugged comes from the high voltage capacitor which usually has a high value bleeder resistor across it that will discharge it over time. But it's always a good idea to make sure it's discharged by shorting it with an insulated screwdriver in the event the bleeder resistor has failed. This applies to microwaves with a linear transformer.