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/
Nah, some parts are build in a way where you have to trust the server for no reason. Sealed sender, Not to make a mitm if you just trust the keys and not validate them