
I have a Raspberry Pi running MPD with a HiFiBerry Digi+ Standard; the optical out goes to an amp in one room, and the coax goes to an amp in another room. (So those two always play the same thing.)
For each additional room, I add another Raspberry Pi with access to the music on the primary Raspberry Pi via sshfs, running MPD, with either another HiFiBerry Digi+ or a USB DAC, and connect to the amplifier in that room. Each room added this way plays different music.
If you wanted all the rooms to have the same thing playing, you could add multiple USB DACs to a single Raspberry Pi, each going somewhere else, MPD can output to all of them. It is probably better to have longer USB runs than longer analog audio runs. Or you can run the analog audio over shielded cat6.
I have not tried adding a Bluetooth input to the Raspberry PI, but it seems like that would be possible with pipewire or jack.





You can’t.
No.
Use https://www.passwordstore.org/. Sync it to a trusted person’s git server. Put the gpg key on a usb stick and give it to them. Write the password to the GPG key on a piece of paper and give it to your lawyer with instructions to give it to your trusted person in the event of your death.