Recently discovered this. Molly supports link with existing device just like on signal desktop. It even has benefit of getting entire chat history unlike signal desktop. Just restore the signal backup file during setup and then click link with existing device. Then scan with you primary phone. Beauty of open source. Molly: https://molly.im/
Good question, check with the signal github
I don’t know man, seems to me XMPP is more secure (unless you trust Signal) and simple to use because you have to jump less hoops