Just a few questions on Full fibre.
2. How can I get another phone in another room to work?
As you are on Full fibre the existing master phone socket and it's extensions are not needed for internet or phoning, so I think you should be able to connect a lead from the hub to the master socket which would then mean the extensions can be used.
I'm no expert on this so am not sure if any rewiring of the master socket is needed.
Yes, that’s the issue, if done incorrectly, the VOIP output could feed back down the external copper pair to wherever that goes, not ideal at all, and may well cause faults for others or voice issues on your exertion wiring. The incoming pair needs dissing off.
One good thing about music, when it hits you feel no pain.