After turning off all electronica in main and guest houses, go to your house circuit breaker and switch off the master circuit breaker.
If both the guest house and main house power circuits go dark then you can test further.
Then, if you have an electrically-saavy friend with a Fox and Hounds circuit tracer, invite them over for a beer or six. Before the beer is opened, they can inject an audio tone (with the 'Fox' part of the set) on the circuit leg from the PC, and if the 'Hound' (amplified speaker) can hear it in the guest house, then it may, repeat may, work. If it cannot be heard, it won't work.
Why the uncertainty? Because power line networking requires being on the same phase leg of the same circuit, and with a Fox and Hounds set, you don't know if you're on the same phase leg.