Failed plug eh? Did the check engine light come on before the overheating?
Check the codes. Might just be about misfire . still good to know.
Helpful to have a camera readily recording the dash CEL/MIL if you use that method for reading.
Try to fish any remaining pieces of that failed spark plug out of the cylinder as anything not small enough to easily exit a valve can cause damage on piston or valve /valve seat. Or cause a valve to stay open and cause worse damage on interference engines (not an issue here) . Easiest way to get non magnetic debris out of a cylinder that I know of (without head disassembly) is to raise the piston with crankshaft and use compressed air in spark port. Other forum contributors may have better solution.
You mention hard-to-start. Was this the case before the overheat? Any issues you can think of before the overheat? How long/how many miles ago was the last time you verified your radiator to be filled?
Since your radiator is empty, and you don't know (or just haven't confirmed) where the coolant exited, a radiator pressure test is in order. Kits are free to borrow with deposit in my area, don't know about yours, ?Tennesee¿ .Look at the gasket on your radiator cap. if it is very cracked or deeply deformed, that's suspect.
also, any pics? Probably irrelevent but where you say "electrode" being burnt out are you talking about the hooked part (anode) or the center (cathode) part?